Operation modes for adjustable resistance bit line structures

ABSTRACT

Methods for reducing leakage currents through unselected memory cells of a memory array during a memory operation are described. In some cases, the leakage currents through the unselected memory cells of the memory array may be reduced by setting an adjustable resistance bit line structure connected to the unselected memory cells into a non-conducting state. The adjustable resistance bit line structure may comprise a bit line structure in which the resistance of an intrinsic (or near intrinsic) polysilicon portion of the bit line structure may be adjusted via an application of a voltage to a select gate portion of the bit line structure that is not directly connected to the intrinsic polysilicon portion. The intrinsic polysilicon portion may be set into a conducting state or a non-conducting state based on the voltage applied to the select gate portion.

CLAIM OF PRIORITY

The present application claims priority to U.S. Provisional ApplicationNo. 62/000,967, entitled “Intrinsic Vertical Bit Line Architecture,”filed May 20, 2014, and claims priority to U.S. Provisional ApplicationNo. 62/041,138, entitled “Intrinsic Vertical Bit Line Architecture,”filed Aug. 24, 2014, both of which are herein incorporated by referencein their entirety.

BACKGROUND

Semiconductor memory is widely used in various electronic devices suchas mobile computing devices, mobile phones, solid-state drives, digitalcameras, personal digital assistants, medical electronics, servers, andnon-mobile computing devices. Semiconductor memory may comprisenon-volatile memory or volatile memory. A non-volatile memory deviceallows information to be stored or retained even when the non-volatilememory device is not connected to a source of power (e.g., a battery).Examples of non-volatile memory include flash memory (e.g., NAND-typeand NOR-type flash memory), Electrically Erasable Programmable Read-OnlyMemory (EEPROM), ferroelectric memory (e.g., FeRAM), magnetoresistivememory (e.g., MRAM), and phase change memory (e.g., PRAM). In recentyears, non-volatile memory devices have been scaled in order to reducethe cost per bit. However, as process geometries shrink, many design andprocess challenges are presented. These challenges include increasedvariability in memory cell I-V characteristics over process, voltage,and temperature variations and increased leakage currents throughunselected memory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1H depict various embodiments of a memory system.

FIGS. 2A-2B depict various embodiments of a portion of athree-dimensional memory array.

FIGS. 3A-3B depict embodiments of a cross-point memory array.

FIGS. 4A-4B depict various embodiments of a portion of athree-dimensional memory array.

FIG. 5 depicts one embodiment of a read/write circuit.

FIGS. 6A-6W depict various embodiments of adjustable resistance bit linestructures within a memory array.

FIGS. 7A-7J depict various embodiments of adjustable resistance bit linestructures within a memory array.

FIGS. 8A-8C depict various embodiments of memory arrays that includeselect gate lines for controlling adjustable resistance bit linestructures.

FIGS. 8D-8G depict various embodiments of a portion of a memory arrayduring a memory operation.

FIG. 8H depicts a flowchart describing one embodiment of a process forperforming a memory operation.

FIG. 8I depicts a flowchart describing one embodiment of a process forperforming a read operation.

FIG. 8J depicts a flowchart describing one embodiment of a process forperforming a programming operation.

FIGS. 8K-8Q depict various embodiments of adjustable resistance bit linestructures during a RESET operation.

FIGS. 8R-8T depict various embodiments of adjustable resistance bit linestructures during a SET operation.

FIGS. 8U-8W depict various embodiments of adjustable resistance bit linestructures during a read operation.

FIG. 9A depicts one embodiment of a circuit for generating unselectedword line voltages.

FIGS. 9B-9C depict a flowchart describing one embodiment of a processfor performing a programming operation.

FIG. 9D depicts a flowchart describing one embodiment of a process forperforming a read operation.

FIGS. 10A-10X depict various embodiments of cross-sectional viewsrelated to processes for fabricating portions of an adjustableresistance bit line structure.

DETAILED DESCRIPTION

Technology is described for reducing leakage currents through unselectedmemory cells (e.g., H-cells and/or U-cells) of a memory array during amemory operation (e.g., a read operation, a programming operation, anerase operation, a program verify operation, or an erase verifyoperation). In some embodiments, leakage currents through unselectedmemory cells of the memory array may be reduced by setting an adjustableresistance bit line structure connected to the unselected memory cellsinto a high resistance state or a non-conducting state. The adjustableresistance bit line structure may comprise a bit line structure in whichthe resistance of an intrinsic (or near intrinsic) polysilicon portionof the bit line structure may be adjusted via an application of avoltage to a select gate (SG) portion of the bit line structure that isisolated or separated from the intrinsic polysilicon portion (e.g., anoxide layer or a gate dielectric layer may be arranged between theintrinsic polysilicon portion and the select gate portion of the bitline structure). In this case, the intrinsic polysilicon portion may beset into a conducting state or a non-conducting state based on thevoltage applied to the select gate portion of the bit line structure.The adjustable resistance bit line structure may comprise a vertical bitline structure (e.g., a bit line structure that is arranged in adirection that is substantially orthogonal to a substrate) or ahorizontal bit line structure (e.g., a bit line structure that isarranged in a direction that is substantially parallel to a substrate).

In one embodiment, a memory array may include a first bit line structureand a second bit line structure. The first bit line structure may beconnected to a first set of memory cells that includes a selected memorycell and the second bit line structure may be connected to a set ofunselected memory cells. In some cases, the first bit line structure andthe second bit line structure may be connected to a global bit line.During a memory operation, the first bit line structure may be set intoa first resistance state and the second bit line structure may be setinto a second resistance state that is greater than the first resistancestate. During the memory operation, the first bit line structure may beset into a conducting state and the second bit line structure may be setinto a non-conducting state. In one example, the first set of memorycells may comprise ReRAM memory cells and the memory operation maycomprise a programming operation or a read operation. In anotherexample, the first set of memory cells may comprise conductive bridgememory cells or programmable metallization memory cells. In some cases,the first bit line structure may include a first semiconducting bodyregion and a first select gate region that is separated from the firstsemiconducting body region by a first gate dielectric. The first bitline structure may be set into a conducting state by applying a firstvoltage to the first select gate region. The second bit line structuremay include a second semiconducting body region and a second select gateregion that is separated from the second semiconducting body region by asecond gate dielectric. The second bit line structure may be set into anon-conducting state by applying a second voltage different from thefirst voltage to the second select gate region.

In some embodiments, a bit line structure may include a distributed FETstructure. The distributed FET structure may comprise a distributed NMOSFET structure or a distributed PMOS FET structure. In some cases, thedistributed FET structure may not include defined drain junctions. Withthe distributed NMOS FET structure, the semiconducting body region ofthe bit line structure may comprise undoped polycrystalline silicon (orpolysilicon), undoped silicon germanium, or undoped indium galliumarsenide. In one embodiment, the carrier concentration of thepolycrystalline silicon may be less than 10̂15 carriers/cm̂3 at 25 C ormay be about 10̂15 carriers/cm̂3 at 25 C or at room temperature. Inanother embodiment, the carrier concentration of the polycrystallinesilicon may be less than 10̂17 carriers/cm̂3 at 25 C or may be about 10̂17carriers/cm̂3 at 25 C or at room temperature. In these cases, a voltagemay be applied to the select gate region of the bit line structure toincrease the carrier concentration in the semiconducting body region andto set the distributed NMOS FET structure into a conducting state. Withthe distributed PMOS FET structure, the semiconducting body region ofthe bit line structure may comprise heavily doped polycrystallinesilicon (e.g., creating a pinch-off FET or causing the bit linestructure to include a pinch-off FET). The carrier concentration of theheavily doped polycrystalline silicon may be greater than 10̂20carriers/cm̂3 at 25 C or may be about 10̂20 carriers/cm̂3 at 25 C or atroom temperature. In this case, a voltage may be applied to the selectgate region of the bit line structure to reduce the carrierconcentration in the semiconducting body region and to set thedistributed PMOS FET structure into a non-conducting state. Thus, a bitline structure may include an intrinsic or undoped semiconducting bodyregion whose resistance may be reduced through the application of avoltage to the select gate region or a bit line structure may include aheavily doped semiconducting body region whose resistance may beincreased through the application of a voltage to the select gateregion.

One issue with having a bit line with high resistance is that sensingmargins may be reduced due to the variability in voltage drops along thehigh resistance bit line during sensing operations. Moreover, a highresistance bit line may cause an increase in the programming voltagesrequired to program a memory cell during programming operations. Thus,using bit lines with high resistance is typically not advisable.

One benefit of using adjustable resistance bit lines or adjustableresistance bit line structures is that leakage currents throughunselected memory cells (e.g., H-cells and/or U-cells) may besignificantly reduced. The reduction in leakage currents may allow forimproved memory array efficiency and for larger memory array sizes.Furthermore, the reduction in leakage currents during memory operationsmay lead to reduced power consumption, reduced energy consumption,improved memory reliability, and/or reduced voltages required to bias amemory array during the memory operations.

In some embodiments, a memory array may comprise a cross-point memoryarray. A cross-point memory array may refer to a memory array in whichtwo-terminal memory cells are placed at the intersections of a first setof control lines (e.g., word lines) arranged in a first direction and asecond set of control lines (e.g., bit lines) arranged in a seconddirection perpendicular to the first direction. The two-terminal memorycells may include a resistance-switching material, such as a phasechange material, a ferroelectric material, or a metal oxide (e.g.,nickel oxide or hafnium oxide). In some cases, each memory cell in across-point memory array may be placed in series with a steering elementor an isolation element, such as a diode, in order to reduce leakagecurrents. In cross-point memory arrays where the memory cells do notinclude an isolation element, controlling and minimizing leakagecurrents may be a significant issue, especially since leakage currentsmay vary greatly over biasing voltage and temperature.

In one embodiment, a non-volatile storage system may include one or moretwo-dimensional arrays of non-volatile memory cells. The memory cellswithin a two-dimensional memory array may form a single layer of memorycells and may be selected via control lines (e.g., word lines and bitlines) in the X and Y directions. In another embodiment, a non-volatilestorage system may include one or more monolithic three-dimensionalmemory arrays in which two or more layers of memory cells may be formedabove a single substrate without any intervening substrates. In somecases, a three-dimensional memory array may include one or more verticalcolumns of memory cells located above and orthogonal to a substrate. Inone example, a non-volatile storage system may include a memory arraywith vertical bit lines or bit lines that are arranged orthogonal to asemiconductor substrate. The substrate may comprise a silicon substrate.The memory array may include rewriteable non-volatile memory cells,wherein each memory cell includes a reversible resistance-switchingelement without an isolation element in series with the reversibleresistance-switching element (e.g., no diode in series with thereversible resistance-switching element).

In some embodiments, a non-volatile storage system may include anon-volatile memory that is monolithically formed in one or morephysical levels of arrays of memory cells having an active area disposedabove a silicon substrate. The non-volatile storage system may alsoinclude circuitry associated with the operation of the memory cells(e.g., decoders, state machines, page registers, or control circuitryfor controlling the reading and/or programming of the memory cells). Thecircuitry associated with the operation of the memory cells may belocated above the substrate or located within the substrate.

In some embodiments, a non-volatile storage system may include amonolithic three-dimensional memory array. The monolithicthree-dimensional memory array may include one or more levels of memorycells. Each memory cell within a first level of the one or more levelsof memory cells may include an active area that is located above asubstrate (e.g., a single-crystal substrate or a crystalline siliconsubstrate). In one example, the active area may include a semiconductorjunction (e.g., a P-N junction). The active area may include a portionof a source or drain region of a transistor. In another example, theactive area may include a channel region of a transistor.

In one embodiment, the memory cells within a memory array may comprisere-writable non-volatile memory cells including a reversibleresistance-switching element. A reversible resistance-switching elementmay include a reversible resistivity-switching material having aresistivity that may be reversibly switched between two or more states.In one embodiment, the reversible resistance-switching material mayinclude a metal oxide (e.g., a binary metal oxide). The metal oxide mayinclude nickel oxide or hafnium oxide. In another embodiment, thereversible resistance-switching material may include a phase changematerial. The phase change material may include a chalcogenide material.In some cases, the re-writeable non-volatile memory cells may compriseresistive RAM (ReRAM) memory cells. In other cases, the re-writeablenon-volatile memory cells may comprise conductive bridge memory cells orprogrammable metallization memory cells.

FIG. 1A depicts one embodiment of a memory system 101 and a host 106.The memory system 101 may comprise a non-volatile storage systeminterfacing with the host (e.g., a mobile computing device or a server).In some cases, the memory system 101 may be embedded within the host106. As examples, the memory system 101 may comprise a memory card, asolid-state drive (SSD) such a high density MLC SSD (e.g., 2-bits/cellor 3-bits/cell) or a high performance SLC SSD, or a hybrid HDD/SSDdrive. As depicted, the memory system 101 includes a memory chipcontroller 105 and a memory chip 102. The memory chip 102 may includevolatile memory and/or non-volatile memory. Although a single memorychip is depicted, the memory system 101 may include more than one memorychip (e.g., four or eight memory chips). The memory chip controller 105may receive data and commands from host 106 and provide memory chip datato host 106. The memory chip controller 105 may include one or morestate machines, page registers, SRAM, and control circuitry forcontrolling the operation of memory chip 102. The one or more statemachines, page registers, SRAM, and control circuitry for controllingthe operation of the memory chip may be referred to as managing orcontrol circuits. The managing or control circuits may facilitate one ormore memory array operations including forming, erasing, programming, orreading operations.

In some embodiments, the managing or control circuits (or a portion ofthe managing or control circuits) for facilitating one or more memoryarray operations may be integrated within the memory chip 102. Thememory chip controller 105 and memory chip 102 may be arranged on asingle integrated circuit or arranged on a single die. In otherembodiments, the memory chip controller 105 and memory chip 102 may bearranged on different integrated circuits. In some cases, the memorychip controller 105 and memory chip 102 may be integrated on a systemboard, logic board, or a PCB.

The memory chip 102 includes memory core control circuits 104 and amemory core 103. Memory core control circuits 104 may include logic forcontrolling the selection of memory blocks (or arrays) within memorycore 103, controlling the generation of voltage references for biasing aparticular memory array into a read or write state, and generating rowand column addresses. The memory core 103 may include one or moretwo-dimensional arrays of memory cells or one or more three-dimensionalarrays of memory cells. In one embodiment, the memory core controlcircuits 104 and memory core 103 may be arranged on a single integratedcircuit. In other embodiments, the memory core control circuits 104 (ora portion of the memory core control circuits) and memory core 103 maybe arranged on different integrated circuits.

Referring to FIG. 1A, a memory operation may be initiated when host 106sends instructions to memory chip controller 105 indicating that itwould like to read data from memory system 101 or write data to memorysystem 101. In the event of a write (or programming) operation, host 106may send to memory chip controller 105 both a write command and the datato be written. The data to be written may be buffered by memory chipcontroller 105 and error correcting code (ECC) data may be generatedcorresponding with the data to be written. The ECC data, which allowsdata errors that occur during transmission or storage to be detectedand/or corrected, may be written to memory core 103 or stored innon-volatile memory within memory chip controller 105. In oneembodiment, the ECC data is generated and data errors are corrected bycircuitry within memory chip controller 105.

Referring to FIG. 1A, the operation of memory chip 102 may be controlledby memory chip controller 105. In one example, before issuing a writeoperation to memory chip 102, memory chip controller 105 may check astatus register to make sure that memory chip 102 is able to accept thedata to be written. In another example, before issuing a read operationto memory chip 102, memory chip controller 105 may pre-read overheadinformation associated with the data to be read. The overheadinformation may include ECC data associated with the data to be read ora redirection pointer to a new memory location within memory chip 102 inwhich to read the data requested. Once a read or write operation isinitiated by memory chip controller 105, memory core control circuits104 may generate the appropriate bias voltages for word lines and bitlines within memory core 103, as well as generate the appropriate memoryblock, row, and column addresses.

In some embodiments, one or more managing or control circuits may beused for controlling the operation of a memory array within the memorycore 103. The one or more managing or control circuits may providecontrol signals to a memory array in order to perform a read operationand/or a write operation on the memory array. In one example, the one ormore managing or control circuits may include any one of or acombination of control circuitry, state machines, decoders, senseamplifiers, read/write circuits, and/or controllers. The one or moremanaging circuits may perform or facilitate one or more memory arrayoperations including erasing, programming, or reading operations. In oneexample, one or more managing circuits may comprise an on-chip memorycontroller for determining row and column address, word line and bitline addresses, memory array enable signals, and data latching signals.

FIG. 1B depicts one embodiment of memory core control circuits 104. Asdepicted, the memory core control circuits 104 include address decoders170, voltage generators for selected control lines 172, and voltagegenerators for unselected control lines 174. Control lines may includeword lines, bit lines, or a combination of word lines and bit lines.Selected control lines may include selected word lines or selected bitlines that are used to place memory cells into a selected state.Unselected control lines may include unselected word lines or unselectedbit lines that are used to place memory cells into an unselected state.The voltage generators (or voltage regulators) for selected controllines 172 may comprise one or more voltage generators for generatingselected control line voltages. The voltage generators for unselectedcontrol lines 174 may comprise one or more voltage generators forgenerating unselected control line voltages. Address decoders 170 maygenerate memory block addresses, as well as row addresses and columnaddresses for a particular memory block.

FIGS. 1C-1F depict one embodiment of a memory core organization thatincludes a memory core having multiple memory bays, and each memory bayhaving multiple memory blocks. Although a memory core organization isdisclosed where memory bays comprise memory blocks, and memory blockscomprise a group of memory cells, other organizations or groupings canalso be used with the technology described herein.

FIG. 1C depicts one embodiment of memory core 103 in FIG. 1A. Asdepicted, memory core 103 includes memory bay 330 and memory bay 331. Insome embodiments, the number of memory bays per memory core can bedifferent for different implementations. For example, a memory core mayinclude only a single memory bay or a plurality of memory bays (e.g., 16memory bays or 256 memory bays).

FIG. 1D depicts one embodiment of memory bay 330 in FIG. 1C. Asdepicted, memory bay 330 includes memory blocks 310-312 and read/writecircuits 306. In some embodiments, the number of memory blocks permemory bay may be different for different implementations. For example,a memory bay may include one or more memory blocks (e.g., 32 memoryblocks per memory bay). Read/write circuits 306 include circuitry forreading and writing memory cells within memory blocks 310-312. Asdepicted, the read/write circuits 306 may be shared across multiplememory blocks within a memory bay. This allows chip area to be reducedsince a single group of read/write circuits 306 may be used to supportmultiple memory blocks. However, in some embodiments, only a singlememory block may be electrically coupled to read/write circuits 306 at aparticular time to avoid signal conflicts.

In some embodiments, read/write circuits 306 may be used to write one ormore pages of data into the memory blocks 310-312 (or into a subset ofthe memory blocks). The memory cells within the memory blocks 310-312may permit direct over-writing of pages (i.e., data representing a pageor a portion of a page may be written into the memory blocks 310-312without requiring an erase or reset operation to be performed on thememory cells prior to writing the data). In one example, the memorysystem 101 in FIG. 1A may receive a write command including a targetaddress and a set of data to be written to the target address. Thememory system 101 may perform a read-before-write (RBW) operation toread the data currently stored at the target address before performing awrite operation to write the set of data to the target address. Thememory system 101 may then determine whether a particular memory cellmay stay at its current state (i.e., the memory cell is already at thecorrect state), needs to be set to a “0” state, or needs to be reset toa “1” state. The memory system 101 may then write a first subset of thememory cells to the “0” state and then write a second subset of thememory cells to the “1” state. The memory cells that are already at thecorrect state may be skipped over, thereby improving programming speedand reducing the cumulative voltage stress applied to unselected memorycells. A particular memory cell may be set to the “1” state by applyinga first voltage difference across the particular memory cell of a firstpolarity (e.g., +1.5V). The particular memory cell may be reset to the“0” state by applying a second voltage difference across the particularmemory cell of a second polarity that is opposite to that of the firstpolarity (e.g., −1.5V).

In some cases, read/write circuits 306 may be used to program aparticular memory cell to be in one of three or more data/resistancestates (i.e., the particular memory cell may comprise a multi-levelmemory cell). In one example, the read/write circuits 306 may apply afirst voltage difference (e.g., 2V) across the particular memory cell toprogram the particular memory cell into a first state of the three ormore data/resistance states or a second voltage difference (e.g., 1V)across the particular memory cell that is less than the first voltagedifference to program the particular memory cell into a second state ofthe three or more data/resistance states. Applying a smaller voltagedifference across the particular memory cell may cause the particularmemory cell to be partially programmed or programmed at a slower ratethan when applying a larger voltage difference. In another example, theread/write circuits 306 may apply a first voltage difference across theparticular memory cell for a first time period (e.g., 150 ns) to programthe particular memory cell into a first state of the three or moredata/resistance states or apply the first voltage difference across theparticular memory cell for a second time period less than the first timeperiod (e.g., 50 ns). One or more programming pulses followed by amemory cell verification phase may be used to program the particularmemory cell to be in the correct state.

FIG. 1E depicts one embodiment of memory block 310 in FIG. 1D. Asdepicted, memory block 310 includes a memory array 301, row decoder 304,and column decoder 302. Memory array 301 may comprise a contiguous groupof memory cells having contiguous word lines and bit lines. Memory array301 may comprise one or more layers of memory cells. Memory array 310may comprise a two-dimensional memory array or a three-dimensionalmemory array. The row decoder 304 decodes a row address and selects aparticular word line in memory array 301 when appropriate (e.g., whenreading or writing memory cells in memory array 301). The column decoder302 decodes a column address and selects a particular group of bit linesin memory array 301 to be electrically coupled to read/write circuits,such as read/write circuits 306 in FIG. 1D. In one embodiment, thenumber of word lines is 4K per memory layer, the number of bit lines is1K per memory layer, and the number of memory layers is 4, providing amemory array 301 containing 16M memory cells.

FIG. 1F depicts one embodiment of a memory bay 332. Memory bay 332 isone example of an alternative implementation for memory bay 330 in FIG.1D. In some embodiments, row decoders, column decoders, and read/writecircuits may be split or shared between memory arrays. As depicted, rowdecoder 349 is shared between memory arrays 352 and 354 because rowdecoder 349 controls word lines in both memory arrays 352 and 354 (i.e.,the word lines driven by row decoder 349 are shared). Row decoders 348and 349 may be split such that even word lines in memory array 352 aredriven by row decoder 348 and odd word lines in memory array 352 aredriven by row decoder 349. Column decoders 344 and 346 may be split suchthat even bit lines in memory array 352 are controlled by column decoder346 and odd bit lines in memory array 352 are driven by column decoder344. The selected bit lines controlled by column decoder 344 may beelectrically coupled to read/write circuits 340. The selected bit linescontrolled by column decoder 346 may be electrically coupled toread/write circuits 342. Splitting the read/write circuits intoread/write circuits 340 and 342 when the column decoders are split mayallow for a more efficient layout of the memory bay.

FIG. 1G depicts one embodiment of a schematic diagram (including wordlines and bit lines) corresponding with memory bay 332 in FIG. 1F. Asdepicted, word lines WL1, WL3, and WL5 are shared between memory arrays352 and 354 and controlled by row decoder 349 of FIG. 1F. Word linesWL0, WL2, WL4, and WL6 are driven from the left side of memory array 352and controlled by row decoder 348 of FIG. 1F. Word lines WL14, WL16,WL18, and WL20 are driven from the right side of memory array 354 andcontrolled by row decoder 350 of FIG. 1F. Bit lines BL0, BL2, BL4, andBL6 are driven from the bottom of memory array 352 and controlled bycolumn decoder 346 of FIG. 1F. Bit lines BL1, BL3, and BL5 are drivenfrom the top of memory array 352 and controlled by column decoder 344 ofFIG. 1F.

In one embodiment, the memory arrays 352 and 354 may comprise memorylayers that are oriented in a horizontal plane that is horizontal to thesupporting substrate. In another embodiment, the memory arrays 352 and354 may comprise memory layers that are oriented in a vertical planethat is vertical with respect to the supporting substrate (i.e., thevertical plane is perpendicular to the supporting substrate). In thiscase, the bit lines of the memory arrays may comprise vertical bitlines.

FIG. 1H depicts one embodiment of a schematic diagram (including wordlines and bit lines) corresponding with a memory bay arrangement whereinword lines and bit lines are shared across memory blocks, and both rowdecoders and column decoders are split. Sharing word lines and/or bitlines helps to reduce layout area since a single row decoder and/orcolumn decoder can be used to support two memory arrays. As depicted,word lines WL1, WL3, and WL5 are shared between memory arrays 406 and408. Bit lines BL1, BL3, and BL5 are shared between memory arrays 406and 402. Row decoders are split such that word lines WL0, WL2, WL4, andWL6 are driven from the left side of memory array 406 and word linesWL1, WL3, and WL5 are driven from the right side of memory array 406.Column decoders are split such that bit lines BL0, BL2, BL4, and BL6 aredriven from the bottom of memory array 406 and bit lines BL1, BL3, andBL5 are driven from the top of memory array 406. Splitting row and/orcolumn decoders also helps to relieve layout constraints (e.g., thecolumn decoder pitch can be relieved by 2× since the split columndecoders need only drive every other bit line instead of every bitline).

FIG. 2A depicts one embodiment of a portion of a monolithicthree-dimensional memory array 201 that includes a second memory level220 positioned above a first memory level 218. Memory array 201 is oneexample of an implementation for memory array 301 in FIG. 1E. The bitlines 206 and 210 are arranged in a first direction and the word lines208 are arranged in a second direction perpendicular to the firstdirection. As depicted, the upper conductors of first memory level 218may be used as the lower conductors of the second memory level 220 thatis positioned above the first memory level. In a memory array withadditional layers of memory cells, there would be correspondingadditional layers of bit lines and word lines.

As depicted in FIG. 2A, memory array 201 includes a plurality of memorycells 200. The memory cells 200 may include re-writeable memory cells.The memory cells 200 may include non-volatile memory cells or volatilememory cells. With respect to first memory level 218, a first portion ofmemory cells 200 are between and connect to bit lines 206 and word lines208. With respect to second memory level 220, a second portion of memorycells 200 are between and connect to bit lines 210 and word lines 208.In one embodiment, each memory cell includes a steering element (e.g., adiode) and a memory element (i.e., a state change element). In oneexample, the diodes of the first memory level 218 may be upward pointingdiodes as indicated by arrow A₁ (e.g., with p regions at the bottom ofthe diodes), while the diodes of the second memory level 220 may bedownward pointing diodes as indicated by arrow A₂ (e.g., with n regionsat the bottom of the diodes), or vice versa. In another embodiment, eachmemory cell includes a state change element and does not include asteering element. The absence of a diode (or other steering element)from a memory cell may reduce the process complexity and costsassociated with manufacturing a memory array.

In one embodiment, the memory cells 200 of FIG. 2A may comprisere-writable non-volatile memory cells including a reversibleresistance-switching element. A reversible resistance-switching elementmay include a reversible resistivity-switching material having aresistivity that may be reversibly switched between two or more states.In one embodiment, the reversible resistance-switching material mayinclude a metal oxide (e.g., a binary metal oxide). The metal oxide mayinclude nickel oxide or hafnium oxide. In another embodiment, thereversible resistance-switching material may include a phase changematerial. The phase change material may include a chalcogenide material.In some cases, the re-writeable non-volatile memory cells may compriseresistive RAM (ReRAM) devices.

In another embodiment, the memory cells 200 of FIG. 2A may includeconductive bridge memory elements. A conductive bridge memory elementmay also be referred to as a programmable metallization cell. Aconductive bridge memory element may be used as a state change elementbased on the physical relocation of ions within a solid electrolyte. Insome cases, a conductive bridge memory element may include two solidmetal electrodes, one relatively inert (e.g., tungsten) and the otherelectrochemically active (e.g., silver or copper), with a thin film ofthe solid electrolyte between the two electrodes. As temperatureincreases, the mobility of the ions also increases causing theprogramming threshold for the conductive bridge memory cell to decrease.Thus, the conductive bridge memory element may have a wide range ofprogramming thresholds over temperature.

Referring to FIG. 2A, in one embodiment of a read operation, the datastored in one of the plurality of memory cells 200 may be read bybiasing one of the word lines (i.e., the selected word line) to aselected word line voltage in read mode (e.g., 0V). A read circuit maythen be used to bias a selected bit line connected to the selectedmemory cell to the selected bit line voltage in read mode (e.g., 1.0V).In some cases, in order to avoid sensing leakage current from the manyunselected word lines to the selected bit line, the unselected wordlines may be biased to the same voltage as the selected bit lines (e.g.,1.0V). To avoid leakage current from the selected word line to theunselected bit lines, the unselected bit lines may be biased to the samevoltage as the selected word line (e.g., 0V); however, biasing theunselected word lines to the same voltage as the selected bit lines andbiasing the unselected bit lines to the same voltage as the selectedword line may place a substantial voltage stress across the unselectedmemory cells driven by both the unselected word lines and the unselectedbit lines.

In an alternative read biasing scheme, both the unselected word linesand the unselected bit lines may be biased to an intermediate voltagethat is between the selected word line voltage and the selected bit linevoltage. Applying the same voltage to both the unselected word lines andthe unselected bit lines may reduce the voltage stress across theunselected memory cells driven by both the unselected word lines and theunselected bit lines; however, the reduced voltage stress comes at theexpense of increased leakage currents associated with the selected wordline and the selected bit line. Before the selected word line voltagehas been applied to the selected word line, the selected bit linevoltage may be applied to the selected bit line, and a read circuit maythen sense an auto zero amount of current through the selected memorybit line which is subtracted from the bit line current in a secondcurrent sensing when the selected word line voltage is applied to theselected word line. The leakage current may be subtracted out by usingthe auto zero current sensing.

Referring to FIG. 2A, in one embodiment of a write operation, thereversible resistance-switching material may be in an initialhigh-resistivity state that is switchable to a low-resistivity stateupon application of a first voltage and/or current. Application of asecond voltage and/or current may return the reversibleresistance-switching material back to the high-resistivity state.Alternatively, the reversible resistance-switching material may be in aninitial low-resistance state that is reversibly switchable to ahigh-resistance state upon application of the appropriate voltage(s)and/or current(s). When used in a memory cell, one resistance state mayrepresent a binary data “0” while another resistance state may representa binary data “1.” In some cases, a memory cell may be considered tocomprise more than two data/resistance states (i.e., a multi-levelmemory cell). In some cases, a write operation may be similar to a readoperation except with a larger voltage range placed across the selectedmemory cells.

The process of switching the resistance of a reversibleresistance-switching element from a high-resistivity state to alow-resistivity state may be referred to as SETTING the reversibleresistance-switching element. The process of switching the resistancefrom the low-resistivity state to the high-resistivity state may bereferred to as RESETTING the reversible resistance-switching element.The high-resistivity state may be associated with binary data “1” andthe low-resistivity state may be associated with binary data “0.” Inother embodiments, SETTING and RESETTING operations and/or the dataencoding may be reversed. For example, the high-resistivity state may beassociated with binary data “0” and the low-resistivity state may beassociated with binary data “1.” In some embodiments, a higher thannormal programming voltage may be required the first time a reversibleresistance-switching element is SET into the low-resistivity state asthe reversible resistance-switching element may have been placed into aresistance state that is higher than the high-resistivity state whenfabricated. The term “FORMING” may refer to the setting of a reversibleresistance-switching element into a low-resistivity state for the firsttime after fabrication or the resetting of a reversibleresistance-switching element into a high-resistivity state for the firsttime after fabrication. In some cases, after a FORMING operation or amemory cell preconditioning operation has been performed, the reversibleresistance-switching element may be RESET to the high-resistivity stateand then SET again to the low-resistivity state.

Referring to FIG. 2A, in one embodiment of a write operation, data maybe written to one of the plurality of memory cells 200 by biasing one ofthe word lines (i.e., the selected word line) to the selected word linevoltage in write mode (e.g., 5V). A write circuit may be used to biasthe bit line connected to the selected memory cell to the selected bitline voltage in write mode (e.g., 0V). In some cases, in order toprevent program disturb of unselected memory cells sharing the selectedword line, the unselected bit lines may be biased such that a firstvoltage difference between the selected word line voltage and theunselected bit line voltage is less than a first disturb threshold. Toprevent program disturb of unselected memory cells sharing the selectedbit line, the unselected word lines may be biased such that a secondvoltage difference between the unselected word line voltage and theselected bit line voltage is less than a second disturb threshold. Thefirst disturb threshold and the second disturb threshold may bedifferent depending on the amount of time in which the unselected memorycells susceptible to disturb are stressed.

In one write biasing scheme, both the unselected word lines and theunselected bit lines may be biased to an intermediate voltage that isbetween the selected word line voltage and the selected bit linevoltage. The intermediate voltage may be generated such that a firstvoltage difference across unselected memory cells sharing a selectedword line is greater than a second voltage difference across otherunselected memory cells sharing a selected bit line. One reason forplacing the larger voltage difference across the unselected memory cellssharing a selected word line is that the memory cells sharing theselected word line may be verified immediately after a write operationin order to detect a write disturb.

FIG. 2B depicts a subset of the memory array and routing layers of oneembodiment of a three-dimensional memory array, such as memory array 301in FIG. 1E. As depicted, the Memory Array layers are positioned abovethe Substrate. The Memory Array layers include bit line layers BL0, BL1and BL2, and word line layers WL0 and WL1. In other embodiments,additional bit line and word line layers can also be implemented.Supporting circuitry (e.g., row decoders, column decoders, andread/write circuits) may be arranged on the surface of the Substratewith the Memory Array layers fabricated above the supporting circuitry.An integrated circuit implementing a three-dimensional memory array mayalso include multiple metal layers for routing signals between differentcomponents of the supporting circuitry, and between the supportingcircuitry and the bit lines and word lines of the memory array. Theserouting layers can be arranged above the supporting circuitry that isimplemented on the surface of the Substrate and below the Memory Arraylayers.

As depicted in FIG. 2B, two metal layers R1 and R2 may be used forrouting layers; however, other embodiments can include more or less thantwo metal layers. In one example, these metal layers R1 and R2 may beformed of tungsten (about 1 ohm/square). Positioned above the MemoryArray layers may be one or more top metal layers used for routingsignals between different components of the integrated circuit, such asthe Top Metal layer. In one example, the Top Metal layer is formed ofcopper or aluminum (about 0.05 ohms/square), which may provide a smallerresistance per unit area than metal layers R1 and R2. In some cases,metal layers R1 and R2 may not be implemented using the same materialsas those used for the Top Metal layers because the metal used for R1 andR2 must be able to withstand the processing steps for fabricating theMemory Array layers on top of R1 and R2 (e.g., satisfying a particularthermal budget during fabrication).

FIG. 3A depicts one embodiment of a cross-point memory array 360. In oneexample, the cross-point memory array 360 may correspond with memoryarray 201 in FIG. 2A. As depicted, cross-point memory array 360 includesword lines 365-368 and bit lines 361-364. The bit lines 361 may comprisevertical bit lines or horizontal bit lines. Word line 366 comprises aselected word line and bit line 362 comprises a selected bit line. Atthe intersection of selected word line 366 and selected bit line 362 isa selected memory cell (an S cell). The voltage across the S cell is thedifference between the selected word line voltage and the selected bitline voltage. Memory cells at the intersections of the selected wordline 366 and the unselected bit lines 361, 363, and 364 compriseunselected memory cells (H cells). H cells are unselected memory cellsthat share a selected word line that is biased to the selected word linevoltage. The voltage across the H cells is the difference between theselected word line voltage and the unselected bit line voltage. Memorycells at the intersections of the selected bit line 362 and theunselected word lines 365, 367, and 368 comprise unselected memory cells(F cells). F cells are unselected memory cells that share a selected bitline that is biased to a selected bit line voltage. The voltage acrossthe F cells is the difference between the unselected word line voltageand the selected bit line voltage. Memory cells at the intersections ofthe unselected word lines 365, 367, and 368 and the unselected bit lines361, 363, and 364 comprise unselected memory cells (U cells). Thevoltage across the U cells is the difference between the unselected wordline voltage and the unselected bit line voltage.

The number of F cells is related to the length of the bit lines (or thenumber of memory cells connected to a bit line) while the number of Hcells is related to the length of the word lines (or the number ofmemory cells connected to a word line). The number of U cells is relatedto the product of the word line length and the bit line length. In oneembodiment, each memory cell sharing a particular word line, such asword line 365, may be associated with a particular page stored withinthe cross-point memory array 360.

FIG. 3B depicts an alternative embodiment of a cross-point memory array370. In one example, the cross-point memory array 370 may correspondwith memory array 201 in FIG. 2A. As depicted, cross-point memory array370 includes word lines 375-378 and bit lines 371-374. The bit lines 361may comprise vertical bit lines or horizontal bit lines. Word line 376comprises a selected word line and bit lines 372 and 374 compriseselected bit lines. Although both bit lines 372 and 374 are selected,the voltages applied to bit line 372 and bit line 374 may be different.For example, in the case that bit line 372 is associated with a firstmemory cell to be programmed (i.e., an S cell), then bit line 372 may bebiased to a selected bit line voltage in order to program the firstmemory cell. In the case that bit line 374 is associated with a secondmemory cell that is not to be programmed (i.e., an I cell), then bitline 374 may be biased to a program inhibit voltage (i.e., to a bit linevoltage that will prevent the second memory cell from being programmed).

At the intersection of selected word line 376 and selected bit line 374is a program inhibited memory cell (an I cell). The voltage across the Icell is the difference between the selected word line voltage and theprogram inhibit voltage. Memory cells at the intersections of theselected bit line 374 and the unselected word lines 375, 377, and 378comprise unselected memory cells (X cells). X cells are unselectedmemory cells that share a selected bit line that is biased to a programinhibit voltage. The voltage across the X cells is the differencebetween the unselected word line voltage and the program inhibitvoltage. In one embodiment, the program inhibit voltage applied to theselected bit line 374 may be the same as or substantially the same asthe unselected bit line voltage. In another embodiment, the programinhibit voltage may be a voltage that is greater than or less than theunselected bit line voltage. For example, the program inhibit voltagemay be set to a voltage that is between the selected word line voltageand the unselected bit line voltage. In some cases, the program inhibitvoltage applied may be a function of temperature. In one example, theprogram inhibit voltage may track the unselected bit line voltage overtemperature.

In one embodiment, two or more pages may be associated with a particularword line. In one example, word line 375 may be associated with a firstpage and a second page. The first page may correspond with bit lines 371and 373 and the second page may correspond with bit lines 372 and 374.In this case, the first page and the second page may correspond withinterdigitated memory cells that share the same word line. When a memoryarray operation is being performed on the first page (e.g., aprogramming operation) and the selected word line 376 is biased to theselected word line voltage, one or more other pages also associated withthe selected word line 376 may comprise H cells because the memory cellsassociated with the one or more other pages will share the same selectedword line as the first page.

In some embodiments, not all unselected bit lines may be driven to anunselected bit line voltage. Instead, a number of unselected bit linesmay be floated and indirectly biased via the unselected word lines. Inthis case, the memory cells of memory array 370 may comprise resistivememory elements without isolating diodes. In one embodiment, the bitlines 372 and 373 may comprise vertical bit lines in a three dimensionalmemory array comprising comb shaped word lines.

FIG. 4A depicts one embodiment of a portion of a monolithicthree-dimensional memory array 416 that includes a first memory level412 positioned below a second memory level 410. Memory array 416 is oneexample of an implementation for memory array 301 in FIG. 1E. The localbit lines LBL₁₁-LBL₃₃ are arranged in a first direction (i.e., avertical direction) and the word lines WL₁₀-WL₂₃ are arranged in asecond direction perpendicular to the first direction. This arrangementof vertical bit lines in a monolithic three-dimensional memory array isone embodiment of a vertical bit line memory array. As depicted,disposed between the intersection of each local bit line and each wordline is a particular memory cell (e.g., memory cell M₁₁₁ is disposedbetween local bit line LBL₁₁ and word line WL₁₀). In one example, theparticular memory cell may include a floating gate device or a chargetrap device (e.g., using a silicon nitride material). In anotherexample, the particular memory cell may include a reversibleresistance-switching material, a metal oxide, a phase change material,or a ReRAM material. The global bit lines GBL₁-GBL₃ are arranged in athird direction that is perpendicular to both the first direction andthe second direction. A set of bit line select devices (e.g., Q₁₁-Q₃₁)may be used to select a set of local bit lines (e.g., LBL₁₁-LBL₃₁). Asdepicted, bit line select devices Q₁₁-Q₃₁ are used to select the localbit lines LBL₁₁-LBL₃₁ and to connect the local bit lines LBL₁₁-LBL₃₁ tothe global bit lines GBL₁-GBL₃ using row select line SG₁. Similarly, bitline select devices Q₁₂-Q₃₂ are used to selectively connect the localbit lines LBL₁₂-LBL₃₂ to the global bit lines GBL₁-GBL₃ using row selectline SG₂ and bit line select devices Q₁₃-Q₃₃ are used to selectivelyconnect the local bit lines LBL₁₃-LBL₃₃ to the global bit linesGBL₁-GBL₃ using row select line SG₃.

Referring to FIG. 4A, as only a single bit line select device is usedper local bit line, only the voltage of a particular global bit line maybe applied to a corresponding local bit line. Therefore, when a firstset of local bit lines (e.g., LBL₁₁-LBL₃₁) is biased to the global bitlines GBL₁-GBL₃, the other local bit lines (e.g., LBL₁₂-LBL₃₂ andLBL₁₃-LBL₃₃) must either also be driven to the same global bit linesGBL₁-GBL₃ or be floated. In one embodiment, during a memory operation,all local bit lines within the memory array are first biased to anunselected bit line voltage by connecting each of the global bit linesto one or more local bit lines. After the local bit lines are biased tothe unselected bit line voltage, then only a first set of local bitlines LBL₁₁-LBL₃₁ are biased to one or more selected bit line voltagesvia the global bit lines GBL₁-GBL₃, while the other local bit lines(e.g., LBL₁₂-LBL₃₂ and LBL₁₃-LBL₃₃) are floated. The one or moreselected bit line voltages may correspond with, for example, one or moreread voltages during a read operation or one or more programmingvoltages during a programming operation.

In one embodiment, a vertical bit line memory array, such as memoryarray 416, includes a greater number of memory cells along the wordlines as compared with the number of memory cells along the vertical bitlines (e.g., the number of memory cells along a word line may be morethan 10 times the number of memory cells along a bit line). In oneexample, the number of memory cells along each bit line may be 16 or 32,while the number of memory cells along each word line may be 2048 ormore than 4096.

FIG. 4B depicts one embodiment of a portion of a monolithicthree-dimensional memory array that includes vertical strips of anon-volatile memory material. The physical structure depicted in FIG. 4Bmay comprise one implementation for a portion of the monolithicthree-dimensional memory array depicted in FIG. 4A. The vertical stripsof non-volatile memory material may be formed in a direction that isperpendicular to a substrate (e.g., in the Z direction). A verticalstrip of the non-volatile memory material 414 may include, for example,a vertical oxide layer, a vertical metal oxide layer (e.g., nickel oxideor hafnium oxide), a vertical layer of phase change material, or avertical charge trapping layer (e.g., a layer of silicon nitride). Thevertical strip of material may comprise a single continuous layer ofmaterial that may be used by a plurality of memory cells or devices. Inone example, portions of the vertical strip of the non-volatile memorymaterial 414 may comprise a part of a first memory cell associated withthe cross section between WL₁₂ and LBL₁₃ and a part of a second memorycell associated with the cross section between WL₂₂ and LBL₁₃. In somecases, a vertical bit line, such as LBL₁₃, may comprise a verticalstructure (e.g., a rectangular prism, a cylinder, or a pillar) and thenon-volatile material may completely or partially surround the verticalstructure (e.g., a conformal layer of phase change material surroundingthe sides of the vertical structure). As depicted, each of the verticalbit lines may be connected to one of a set of global bit lines via aselect transistor. The select transistor may comprise a MOS device(e.g., an NMOS device) or a vertical thin-film transistor (TFT).

FIG. 5 depicts one embodiment of a read/write circuit 502 along with aportion of a memory array 501. Read/write circuit 502 is one example ofan implementation of read/write circuit 306 in FIG. 1D. The portion of amemory array 501 includes two of the many bit lines (one selected bitline labeled “Selected BL” and one unselected bit line labeled“Unselected BL”) and two of the many word lines (one selected word linelabeled “Selected WL” and one unselected word line labeled “UnselectedWL”). The portion of a memory array also includes a selected memory cell550 and unselected memory cells 552-556. In one embodiment, the portionof a memory array 501 may comprise a memory array with bit linesarranged in a direction horizontal to the substrate, such as memoryarray 201 in FIG. 2A. In another embodiment, the portion of a memoryarray 501 may comprise a memory array with bit lines arranged in avertical direction that is perpendicular to the substrate, such asmemory array 416 in FIG. 4A.

As depicted, during a memory array operation (e.g., a programmingoperation), the selected bit line may be biased to 1V, the unselectedword line may be biased to 0.6V, the selected word line may be biased to0V, and the unselected bit line may be biased to 0.5V. In someembodiments, during a second memory array operation, the selected bitline may be biased to a selected bit line voltage (e.g., 2.0V), theunselected word line may be biased to an unselected word line voltage(e.g., 1.0V), the selected word line may be biased to a selected wordline voltage (e.g., 0V), and the unselected bit line may be biased to anunselected bit line voltage (e.g., 1V). In this case, the unselectedmemory cells sharing the selected word line will be biased to thevoltage difference between the selected word line voltage and theunselected bit line voltage. In other embodiments, the memory arraybiasing scheme depicted in FIG. 5 may be reversed such that the selectedbit line is biased to 0V, the unselected word line is biased to 0.4V,the selected word line is biased to 1V, and the unselected bit line isbiased to 0.5V.

As depicted in FIG. 5, the SELB node of read/write circuit 502 may beelectrically coupled to the selected bit line via column decoder 504. Inone embodiment, column decoder 504 may correspond with column decoder302 depicted in FIG. 1E. Transistor 562 couples (or electricallyconnects) node SELB to the Vsense node. The transistor 562 may comprisea low VT nMOS device. Clamp control circuit 564 controls the gate oftransistor 562. The Vsense node is connected to reference current Irefand one input of sense amplifier 566. The other input of sense amplifier566 receives Vref-read, which is the voltage level used for comparingthe Vsense node voltage in read mode. The output of sense amplifier 566is connected to the data out terminal and to data latch 568. Writecircuit 560 is connected to node SELB, the Data In terminal, and datalatch 568.

In one embodiment, during a read operation, read/write circuit 502biases the selected bit line to the selected bit line voltage in readmode. Prior to sensing data, read/write circuit 502 will precharge theVsense node to 2V (or some other voltage greater than the selected bitline voltage). When sensing data, read/write circuit 502 attempts toregulate the SELB node to the selected bit line voltage (e.g., 1V) viaclamp control circuit 564 and transistor 562 in a source-followerconfiguration. If the current through the selected memory cell 550 isgreater than the read current limit, Iref, then, over time, the Vsensenode will fall below Vref-read (e.g., set to 1.5V) and the senseamplifier 566 will read out a data “0.” Outputting a data “0” representsthat the selected memory cell 550 is in a low resistance state (e.g., aSET state). If the current through the selected memory cell 550 is lessthan Iref, then the Vsense node will stay above Vref-read and the senseamplifier 566 will read out a data “1.” Outputting a data “1” representsthat the selected memory cell 550 is in a high resistance state (e.g., aRESET state). Data latch 568 may latch the output of sense amplifier 566after a time period of sensing the current through the selected memorycell (e.g., after 400 ns).

In one embodiment, during a write operation, if the Data In terminalrequests a data “0” to be written to a selected memory cell, thenread/write circuit 502 may bias SELB to the selected bit line voltagefor programming a data “0” in write mode (e.g., 1.2V for a SEToperation) via write circuit 560. The duration of programming the memorycell may be a fixed time period (e.g., using a fixed-width programmingpulse) or variable (e.g., using a write circuit 560 that senses whethera memory cell has been programmed while programming). If the Data Interminal requests a data “1” to be written, then read/write circuit 502may bias SELB to the selected bit line voltage for programming a data“1” in write mode (e.g., 0V or −1.2V for a RESET operation) via writecircuit 560. In some cases, if a selected memory cell is to maintain itscurrent state, then the write circuit 560 may bias SELB to a programinhibit voltage during write mode. The program inhibit voltage may bethe same as or close to the unselected bit line voltage.

FIG. 6A depicts one embodiment of an adjustable resistance bit linestructure. As depicted, the adjustable resistance bit line structure maycomprise a vertical bit line structure that includes an adjustableresistance local bit line AR_LBL 585 that is arranged in a directionthat is substantially orthogonal to a substrate (e.g., the adjustableresistance local bit line AR_LBL 585 may comprise part of a verticalpillar that is orthogonal to a silicon substrate). The adjustableresistance local bit line AR_LBL 585 may comprise intrinsic (or nearintrinsic) polysilicon. The adjustable resistance bit line structurealso includes a select gate SG 582 and an oxide layer Oxide 583 or otherdielectric layer (e.g., a high-k dielectric layer) that is arrangedbetween the adjustable resistance local bit line AR_LBL 585 and theselect gate SG 582. The oxide layer Oxide 583 is also arranged betweenthe adjustable resistance local bit line AR_LBL 586 and the select gateSG 582. Both the adjustable resistance local bit line AR_LBL 585 and theadjustable resistance local bit line AR_LBL 586 extend to the N+polysilicon layer 588 that may be formed over or abut the global bitline GBL 584. In some cases, the select gate SG 582 and the oxide layerOxide 583 may extend into the N+ polysilicon layer 588 such that thebottom of the select gate SG 582 is below the top of the N+ polysiliconlayer 588. The select gate SG 582 may comprise titanium nitride (TiN) orpolysilicon. The adjustable resistance local bit line AR_LBL 585 and theadjustable resistance local bit line AR_LBL 586 may comprise undopedpolysilicon or lightly doped N-polysilicon. The Oxide layer 583 maycomprise silicon dioxide. The global bit line GBL 584 may comprise TiNor tungsten.

In some cases, the adjustable resistance bit line structure may comprisea vertical pillar. The vertical pillar may comprise a rectangular pillaror a cylindrical pillar. The vertical pillar may be formed by etchingthrough a stack of alternating word line layers and oxide layers (e.g.,etching through layers of TiN or polysilicon that are separated by oxidelayers) to form a rectangular, square, or cylindrical trench (or hole)and then depositing the layers for forming the vertical pillar withinthe trench. In one example, the vertical pillar may be formed bydepositing a ReRAM layer, depositing an intrinsic polysilicon layeradjacent to the ReRAM layer, depositing an oxide layer adjacent to theintrinsic polysilicon layer, and depositing a polysilicon layer adjacentto the oxide layer to form the select gate that extends through at leasta portion of the vertical pillar.

In some embodiments, a plurality of adjustable resistance bit linestructures may be formed by first etching through an alternating stackof word line layers and dielectric layers (e.g., etching through layersof TiN or polysilicon that are separated by oxide layers) to form aplurality of memory holes. The plurality of memory holes may compriserectangular, square, or cylindrical holes. The plurality of memory holesmay be formed by patterning and then removing material using variousetching techniques such as dry etching, wet chemical etching, plasmaetching, or reactive-ion etching (RIE). After the plurality of memoryholes have been created, the layers for forming vertical pillars withinthe plurality of memory holes may be deposited. In one example, thevertical pillars may be formed by depositing a ReRAM layer within thememory holes, depositing an intrinsic polysilicon layer on the ReRAMlayer, depositing an oxide layer on the intrinsic polysilicon layer, anddepositing a polysilicon layer on the oxide layer to form the selectgates of the adjustable resistance bit line structures. The layers ofthe vertical pillars may be deposited using various depositiontechniques such as chemical vapor deposition (CVD), physical vapordeposition (PVD), or atomic layer deposition (ALD).

In one embodiment, the resistance or the conductivity of the adjustableresistance local bit line AR_LBL 585 may be adjusted via an applicationof a voltage to the select gate SG 582. In one example, the adjustableresistance local bit line AR_LBL 585 may be set into either a conductingstate or a non-conducting state based on the voltage applied to theselect gate SG 582. When the adjustable resistance local bit line AR_LBL585 is set into a conducting state, then a low resistance path (e.g.,less than 100 ohm or less than 1 Kohm) or a conducting path may beformed between the memory elements connected to the word lines WL0-WL7and the global bit line GBL 584. When the adjustable resistance localbit line AR_LBL 585 is set into a non-conducting state, then a highresistance path (e.g., more than 1 Gohm or more than 10 Gohm) is placedbetween the memory elements connected to the word lines WL0-WL7 and theglobal bit line GBL 584. In effect, the high resistance path may causethe memory elements corresponding with the word lines WL0-WL7 to beelectrically disconnected from the global bit line GBL 584. Furthermore,each memory element of the memory elements corresponding with the wordlines WL0-WL7 may be electrically disconnected from the other memoryelements (e.g., memory element 581 connected to word line WL7 may beelectrically disconnected from the other memory elements connected toword lines WL6-WL0). Therefore, in the case that the adjustableresistance local bit line AR_LBL 585 has been set into a non-conductingstate and word line WL7 comprises a selected word line that has been setto a selected word line voltage, then the leakage currents (e.g., H-cellleakage currents) from word line WL7 to the other word lines WL6-WL0 viathe memory elements connected to the adjustable resistance local bitline AR_LBL 585 may be significantly reduced or eliminated.

Referring to FIG. 6A, word lines WL0-WL7 are arranged on a first side ofthe vertical bit line structure and word lines WL8-WL15 are arranged ona second side of the vertical bit line structure. The word lines maycomprise TiN, polysilicon, or tungsten (W). The word lines may beisolated from each other using an oxide layer not depicted that isarranged between the word line layers. A first set of memory elementsincluding memory element 581 is arranged between the word lines WL0-WL7and the adjustable resistance local bit line AR_LBL 585. A second set ofmemory elements is arranged between the word lines WL8-WL15 and theadjustable resistance local bit line AR_LBL 586. The memory element 581may comprise a reversible resistance-switching element. As examples, thememory element 581 may include a ReRAM material, a metal oxide, nickeloxide, hafnium oxide, aluminum oxide, tantalum oxide, a phase changematerial, or a chalcogenide material.

In one embodiment, the word lines WL0-WL15 may extend into the pagewhile the global bit line GBL 584 may extend horizontally from left toright. The word lines WL0-WL15 may comprise lines that extend in a firstdirection (e.g., the X direction) and the global bit line GBL 584 maycomprise a line that extends in a second direction (e.g., the Ydirection) that is orthogonal to the first direction. The vertical bitline structure may extend in a third direction (e.g., the Z direction)that is orthogonal to both the first direction and the second direction.

FIG. 6B depicts another embodiment of an adjustable resistance bit linestructure. As depicted, the adjustable resistance bit line structure issimilar to the adjustable resistance bit line structure depicted in FIG.6A except that the Oxide layer 583 does not extend to and abut the N+polysilicon layer 588. As depicted, the Oxide layer 583 does not share acommon boundary with the N+ polysilicon layer 588.

In one embodiment, the resistance or the conductivity of the adjustableresistance local bit line AR_LBL 587 may be adjusted via an applicationof a voltage to the select gate SG 582. In one example, the adjustableresistance local bit line AR_LBL 587 may be set into a conducting stateor a non-conducting state based on the voltage applied to the selectgate SG 582. When the adjustable resistance local bit line AR_LBL 587 isset into a conducting state, then a low resistance path or a conductingpath may be formed between the memory elements corresponding with theword lines WL0-WL15 and the global bit line GBL 584. When the adjustableresistance local bit line AR_LBL 587 is set into a non-conducting state,then a high resistance path is placed between the memory elementscorresponding with the word lines WL0-WL15 and the global bit line GBL584. In effect, the high resistance path may cause the memory elementscorresponding with the word lines WL0-WL15 to be electricallydisconnected from the global bit line GBL 584.

FIG. 6C depicts one embodiment of a first adjustable resistance bit linestructure and a second adjustable resistance bit line structure. Thefirst adjustable resistance bit line structure includes a select gate SG596 and an adjustable resistance local bit line AR_LBL 598. The secondadjustable resistance bit line structure includes a select gate SG 597and an adjustable resistance local bit line AR_LBL 599. The adjustableresistance local bit line AR_LBL 598 and the adjustable resistance localbit line AR_LBL 599 are connected to the global bit line GBL 595 (e.g.,via an N+ polysilicon layer).

In some embodiments, the first adjustable resistance bit line structuremay comprise a first distributed FET structure and the second adjustableresistance bit line structure may comprise a second distributed FETstructure. The first adjustable resistance bit line structure may be setinto a conducting state by applying a first voltage to the select gateSG 596 and the second adjustable resistance bit line structure may beset into a non-conducting state by applying a second voltage differentfrom the first voltage to the select gate SG 597. In the case that thefirst distributed FET structure comprises a distributed NMOS FETstructure, then a positive voltage (e.g., 2V-7V) may be applied to theselect gate SG 596. The positive voltage applied to the select gate SG596 may depend on the type of memory operation being performed (e.g., aRESET operation, a SET operation, or a read operation). The positivevoltage applied to the select gate SG 596 may be higher during a RESEToperation compared with during a SET operation. The positive voltageapplied to the select gate SG 596 may be higher during a SET operationcompared with during a read operation. The second adjustable resistancebit line structure may be set into a non-conducting state by applying 0Vor a negative voltage (e.g., −2V) to the select gate SG 597.

In one embodiment, the first adjustable resistance bit line structuremay be connected to a first set of memory cells that includes memoryelements that are connected to word lines WL0-WL4 and the secondadjustable resistance bit line structure may be connected to a secondset of memory cells that includes memory elements that are connected toword lines WL10-WL14. During a memory operation (e.g., a read operation,a programming operation, an erase operation, a program verify operation,or an erase verify operation), the first adjustable resistance bit linestructure may be set into a conducting state such that a conducting pathexists between the memory elements that are connected to word linesWL0-WL4 and the global bit line GBL 595 and the second adjustableresistance bit line structure may be set into a non-conducting statesuch that a conducting path does not exist between the memory elementsthat are connected to word lines WL10-WL14 and the global bit line GBL595. In this case, the memory elements that are connected to word linesWL10-WL14 are electrically disconnected from the global bit line GBL595.

FIG. 6D depicts one embodiment of an adjustable resistance bit linestructure using a cross-sectional view in the global bit line direction.The adjustable resistance bit line structure depicted in FIG. 6D is oneexample of an implementation of the adjustable resistance bit linestructure depicted in FIG. 6A. The adjustable resistance bit linestructure may connect to a global bit line, such as global bit line 616,at the bottom of the adjustable resistance bit line structure. Asdepicted, the adjustable resistance bit line structure includes a selectgate 618 and a dielectric layer 610 arranged between the select gate 618and an adjustable resistance local bit line 604. The adjustableresistance local bit line 604 may comprise intrinsic or undopedpolysilicon. In some cases, the adjustable resistance local bit line 604may comprise lightly doped N-polysilicon. In other cases, the adjustableresistance local bit line 604 may comprise undoped silicon germanium.The dielectric layer 610 may comprise an oxide layer. The select gate618 may comprise TiN or polysilicon. An SG line 602 connects to theselect gate 618. The SG line 602 may comprise TiN, polysilicon, ortungsten. In some cases, the SG line 602 may extend in the word linedirection (e.g., into the page). In other cases, the SG line 602 mayextend in the global bit line direction (e.g., horizontally from left toright on the page). A ReRAM layer 614 is arranged between the word lines606-609 and the adjustable resistance local bit line 604. The ReRAMlayer 614 may comprise a phase change material, a ferroelectricmaterial, or a metal oxide such as nickel oxide or hafnium oxide. Theadjustable resistance local bit line 604 may connect to a global bitline 616 via an N+ polysilicon layer 612. The global bit line 616 maycomprise TiN or tungsten. In one embodiment, FIG. 6D may depict across-sectional view taken along line Z-Z of FIG. 6E.

FIG. 6E depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 6D. In one example, the cross-sectional view takenalong line X-X of FIG. 6D may comprise a horizontal slice taken througha word line layer that includes word lines 607 and 609.

FIG. 6F depicts one embodiment of a top plan view of a portion of amemory array that includes adjustable resistance bit line structures. Insome cases, the adjustable resistance bit line structures may bearranged in groups of hexagonally close-packed rows. As depicted, theword lines 607 and 609 are arranged in a first direction (e.g.,extending from top to bottom of the page) and the global bit lines 616and 617 are arranged in a second direction orthogonal to the firstdirection (e.g., extending from left to right on the page). The SG lines602 and 601 are arranged in the first direction (i.e., in the word linedirection). In one example, if word line 607 comprises a selected wordline, then only SG line 602 may be selected, only SG line 601 may beselected, or both SG lines 602 and 601 may be selected at the same time.All other SG lines within the memory array may be deselected in order toset the adjustable resistance bit line structures not connected to theselected word line into a non-conducting state, thereby reducing leakagecurrents through unselected memory cells within the memory array.

In one embodiment, only one SG line within a memory array may beselected during a memory operation and the other SG lines that are notselected within the memory array may be deselected during the memoryoperation. In another embodiment, two or more SG lines within the memoryarray may be selected during a memory operation and the other SG linesthat are not selected within the memory array may be deselected duringthe memory operation. Selecting more than one SG line at a time mayrelieve SG line driver pitch constraints; however, selecting a largernumber of SG lines within a memory array at the same time may cause anincrease in leakage currents during a memory operation.

FIG. 6G depicts another embodiment of a top plan view of a portion of amemory array that includes adjustable resistance bit line structures. Asdepicted, the word lines 607 and 609 are arranged in a first direction(e.g., extending from top to bottom of the page) and the global bitlines 616 and 617 are arranged in a second direction orthogonal to thefirst direction (e.g., extending from left to right on the page). The SGlines 592 and 593 are arranged in the second direction (i.e., in theglobal bit line direction). In some cases, during a memory operation onthe memory array (e.g., a programming operation), only a fraction of theglobal bit lines may be selected. In one example, during a programmingoperation, only 8 or 16 global bit lines out of 1024 global bit linesmay be selected. In this case, only the SG lines corresponding with theselected global bit lines may be selected while all other SG lineswithin the memory array may be deselected. In some cases, H-cell leakagecurrents may be substantially reduced or eliminated by settingadjustable resistance bit line structures connected to selected wordlines and unselected global bit lines into non-conducting states.

In some embodiments, the SG lines within a memory array may be arrangedsuch that they extend in the word line direction, in the global bit linedirection, or at an angle relative to the word line direction (e.g.,diagonal SG lines that are at a 45 degree angle from the word linedirection or at a 25 degree angle from the word line direction).

FIG. 6H depicts one embodiment of an adjustable resistance bit linestructure with an integrated bottom resistor. The integrated bottomresistor may improve memory cell reliability, reduce the etch depth forthe memory hole in which the adjustable resistance bit line structuremay be formed, and reduce the aspect ratio for the memory hole. Theadjustable resistance bit line structure may connect to a global bitline, such as global bit line 616, at the bottom of the adjustableresistance bit line structure. As depicted, the adjustable resistancebit line structure includes a select gate 618 and a dielectric layer 610arranged between the select gate 618 and an adjustable resistance localbit line 604. The adjustable resistance local bit line 604 may compriseintrinsic or undoped polysilicon. In some cases, the adjustableresistance local bit line 604 may comprise lightly doped N-polysilicon.The dielectric layer 610 may comprise an oxide layer. The select gate618 may comprise TiN or polysilicon. An SG line 602 connects to theselect gate 618. The SG line 602 may comprise TiN, polysilicon, ortungsten. In some cases, the SG line 602 may extend in the word linedirection (e.g., into the page). In other cases, the SG line 602 mayextend in the global bit line direction (e.g., horizontally from left toright on the page). A ReRAM layer 614 is arranged between the word lines606-609 and the adjustable resistance local bit line 604. The ReRAMlayer 614 may comprise a phase change material, a ferroelectricmaterial, or a metal oxide such as nickel oxide or hafnium oxide. Theadjustable resistance local bit line 604 may connect to a global bitline 616 via an integrated bottom resistor comprising an N+ polysiliconlayer 652, an N-polysilicon layer 653, and an N+ polysilicon layer 654.The N-polysilicon layer 653 may comprise lightly doped polysilicon(e.g., with a carrier concentration of about 10̂18 carriers/cm̂3 at 25 Cor room temperature). The global bit line 616 may comprise TiN ortungsten. In one embodiment, the spacing between word lines (e.g., thespacing between word lines 606 and 607) may comprise 20 nm and thespacing between the top of the global bit line 616 and the top of the N+polysilicon layer 652 may comprise 150 nm or 200 nm. One benefit ofusing the adjustable resistance bit line structure depicted in FIG. 6His that the etch depth for etching memory holes may be reduced. Anotherbenefit of using the adjustable resistance bit line structure depictedin FIG. 6H is that memory cell reliability may be improved as theintegrated resistor may reduce voltage stress across memory cellsconnected to the adjustable resistance local bit line.

FIG. 6I depicts an alternative embodiment of a first adjustableresistance bit line structure and a second adjustable resistance bitline structure. The first adjustable resistance bit line structureincludes a select gate SG 596 and an adjustable resistance local bitline AR_LBL 598. The second adjustable resistance bit line structureincludes a select gate SG 597 and an adjustable resistance local bitline AR_LBL 599. The adjustable resistance local bit line AR_LBL 598 andthe adjustable resistance local bit line AR_LBL 599 are connected to theglobal bit line GBL 595 (e.g., via an N+ polysilicon layer). The selectgate SG 596 connects to a select gate line SGL 193 via a vertical TFTthat is controlled by CSG 191. The select gate SG 597 connects to aselect gate line SGL 194 via a vertical TFT that is controlled by CSG192.

In some embodiments, the first adjustable resistance bit line structuremay comprise a first distributed NMOS FET structure and the secondadjustable resistance bit line structure may comprise a seconddistributed NMOS FET structure. The first adjustable resistance bit linestructure may be set into a conducting state by applying a first voltageto the select gate SG 596 and the second adjustable resistance bit linestructure may be set into a non-conducting state by applying a secondvoltage different from the first voltage to the select gate SG 597. Inone example, the first voltage may be applied to the select gate SG 596by enabling the vertical TFT controlled by CSG 191 to electricallyconnect the select gate line SGL 193 to the select gate SG 596. Thesecond voltage may be applied to the select gate SG 597 by enabling thevertical TFT controlled by CSG 192 to electrically connect the selectgate line SGL 194 to the select gate SG 597. In some cases, a transistor(e.g., an NMOS device, a PMOS device, or a JFET device) may be used toselectively connect a select gate line, such as select gate line SGL194, to a select gate, such as select gate SG 597. In some cases, theselect gate line SGL 194 may set the select gate SG 597 to ground or anegative voltage prior to the first voltage being applied to the selectgate SG 596.

In some embodiments, the select gate SG 597 may be precharged to groundprior applying a selected select gate voltage (e.g., 4V) to the selectgate SG 596. In some embodiments, all select gates within a memory arraymay be set to ground (i.e., 0V) prior to applying a selected select gatevoltage (e.g., 4V) to the select gate SG 596. In some embodiments,select gates associated with a plurality of adjustable resistance bitline structures (e.g., adjustable resistance bit line structures to beset into a non-conducting state) that each connect to a particular wordline (e.g., a word line to be selected) may be precharged to 0V prior tosetting a select gate associated with an adjustable resistance bit linestructure (e.g., an adjustable resistance bit line structure to be setinto a conducting state) that connects to the particular word line to aselected select gate voltage (e.g., 5V). In this case, as the pluralityof adjustable resistance bit line structures that each connect to theparticular word line (e.g., a selected word line) have been set intonon-conducting states, H-cell leakage currents from the particular wordline when the particular word line is selected may be significantlyreduced or eliminated.

In one embodiment, the adjustable resistance local bit line AR_LBL 598may be connected to a first set of memory cells that includes memoryelements that are connected to word lines WL0-WL4 and the adjustableresistance local bit line AR_LBL 599 may be connected to a second set ofmemory cells that includes memory elements that are connected to wordlines WL10-WL14. During a memory operation (e.g., a read operation, aprogramming operation, an erase operation, a program verify operation,or an erase verify operation), the adjustable resistance local bit lineAR_LBL 598 may be set into a conducting state such that a conductingpath exists between the memory elements that are connected to word linesWL0-WL4 and the global bit line GBL 595 and the adjustable resistancelocal bit line AR_LBL 599 may be set into a non-conducting state suchthat a conducting path does not exist between the memory elements thatare connected to word lines WL10-WL14 and the global bit line GBL 595.In this case, the memory elements that are connected to word linesWL10-WL14 are electrically disconnected from the global bit line GBL595. The adjustable resistance local bit line AR_LBL 599 may be set intoa non-conducting state by precharging the select gate SG 597 to 0V priorto setting the adjustable resistance local bit line AR_LBL 598 into aconducting state.

FIG. 6J depicts one embodiment of an adjustable resistance bit linestructure using a cross-sectional view in the global bit line direction.The adjustable resistance bit line structure depicted in FIG. 6J is oneexample of an implementation of the adjustable resistance bit linestructure depicted in FIG. 6A. The adjustable resistance bit linestructure may connect to a global bit line, such as global bit line 616,at the bottom of the adjustable resistance bit line structure. Asdepicted, the adjustable resistance bit line structure includes a selectgate 618 and a dielectric layer 610 arranged between the select gate 618and an adjustable resistance local bit line 604. The adjustableresistance local bit line 604 may comprise intrinsic or undopedpolysilicon. In some cases, the adjustable resistance local bit line 604may comprise lightly doped N-polysilicon. The dielectric layer 610 maycomprise an oxide layer. The select gate 618 may comprise TiN orpolysilicon. An SG line 602 connects to a drain of a vertical TFT formedusing P-polysilicon layer 603. The source of the vertical TFT isconnected to the select gate 618. The vertical TFT may comprise an NMOStransistor. In some cases, the vertical TFT formed using P-polysiliconlayer 603 may use a thinner dielectric layer than the dielectric layer610 (e.g., a gate oxide for the vertical TFT may be thinner than thedielectric layer 610). The SG line 602 may comprise TiN, polysilicon, ortungsten. In some cases, the SG line 602 may extend in the word linedirection (e.g., into the page). A ReRAM layer 614 is arranged betweenthe word lines 606-609 and the adjustable resistance local bit line 604.The ReRAM layer 614 may comprise a phase change material, aferroelectric material, or a metal oxide such as nickel oxide or hafniumoxide. The adjustable resistance local bit line 604 may connect to aglobal bit line 616 via an N+ polysilicon layer 612. The global bit line616 may comprise TiN or tungsten. In one embodiment, FIG. 6J may depicta cross-sectional view taken along line Z-Z of FIG. 6K.

FIG. 6K depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 6J. In one example, the cross-sectional view takenalong line X-X of FIG. 6J may comprise a horizontal slice taken througha word line layer that includes word lines 607 and 609.

FIG. 6L depicts one embodiment of a side view of the adjustableresistance bit line structure depicted in FIG. 6J. As depicted, a gatecontrol line 605 for controlling the gate of the vertical TFT formedusing P-polysilicon layer 603 extends in the global bit line direction.The gate control line 605 may comprise TiN or tungsten. The vertical TFTmay comprise an NMOS device. In some cases, the vertical TFT maycomprise a JFET. In one example, the dielectric layer 610 may comprisean oxide layer that is used as the gate oxide for the vertical TFT. Inanother example, the gate dielectric for the vertical TFT may be thinnerthan the dielectric layer 610 used as the gate dielectric for thevertical select gate.

FIG. 6M depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 6L. In one example, the cross-sectional view takenalong line X-X of FIG. 6L may comprise a horizontal slice taken throughthe vertical TFT at the gate control line layer. In some cases, eachvertical TFT positioned above each select gate may include a wrap-aroundgate. The wrap-around gates for each vertical TFT may be separated fromeach other using a dielectric layer or an oxide layer.

FIG. 6N depicts one embodiment of the adjustable resistance bit linestructure depicted in FIG. 6J using a cross-sectional view in the wordline direction. The adjustable resistance bit line structure depicted inFIG. 6N is one example of an implementation of the adjustable resistancebit line structure depicted in FIG. 6A. As depicted, the SG line 602extends in the word line direction along with word lines 606, 607, and608. The global bit line 616 extends in the global bit line direction(e.g., into the page). The gate control line 605 for controlling thegate of the vertical TFT may also extend in the global bit linedirection.

FIG. 6O depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 6N. In one example, the cross-sectional view takenalong line X-X of FIG. 6N may comprise a horizontal slice taken througha word line layer that includes word line 607 and word line 609. Wordline 609 may comprise a word line that is in the same word line layer asword line 607 and adjacent to word line 607.

FIG. 6P depicts one embodiment of an adjustable resistance bit linestructure. The adjustable resistance bit line structure depicted in FIG.6P is one example of an implementation of the adjustable resistance bitline structure depicted in FIG. 6A. The adjustable resistance bit linestructure may connect to a global bit line, such as global bit line 616,at the bottom of the adjustable resistance bit line structure. Asdepicted, the adjustable resistance bit line structure includes a selectgate 618 and a dielectric layer 610 arranged between the select gate 618and an adjustable resistance local bit line 604. The adjustableresistance local bit line 604 may comprise intrinsic or undopedpolysilicon. In some cases, the adjustable resistance local bit line 604may comprise lightly doped N-polysilicon. The dielectric layer 610 maycomprise an oxide layer. The select gate 618 may comprise TiN orpolysilicon. An SG line 602 connects to a drain of a vertical TFT formedusing P-polysilicon layer 603 and dielectric layer 611. Dielectric layer611 may be thinner than dielectric layer 610. Dielectric layer 611 maycomprise an oxide layer. The source of the vertical TFT is connected tothe select gate 618. The vertical TFT may comprise an NMOS transistor.In some cases, the vertical TFT may comprise a JFET. The gate of thevertical TFT may be controlled by gate control line 605 that extends inthe global bit line direction. The gate control line 605 may compriseTiN or tungsten.

The SG line 602 may comprise TiN, polysilicon, or tungsten. In somecases, the SG line 602 may extend in the word line direction (e.g., intothe page). A ReRAM layer 614 is arranged between the word lines 606-609and the adjustable resistance local bit line 604. The ReRAM layer 614may comprise a phase change material, a ferroelectric material, or ametal oxide such as nickel oxide or hafnium oxide. The adjustableresistance local bit line 604 may connect to a global bit line 616 viaN+ polysilicon layer 612. The global bit line 616 may comprise TiN ortungsten. In one embodiment, the spacing between word line layers (e.g.,the spacing between word lines 606 and 607) may comprise 20 nm and thespacing between the top of the global bit line 616 and the bottom of theword line layer that includes word line 608 may comprise 150 nm or 200nm.

In some embodiments, the distance 613 between the lowest word line layerand the top of the N+ polysilicon layer 612 may be set in order toprovide a particular resistance value or to provide a particular voltagedrop during a memory operation (e.g., during a programming operationsuch as a SET or RESET operation). In some cases, the lowest word linelayer that includes word line 608 may comprise a dummy word line layer.In some cases, the distance 613 between the lowest word line layer andthe top of the N+ polysilicon layer 612 may be more than ten times thespacing between word line layers (e.g., the spacing between the wordline layer that includes word line 606 and the word line layer thatincludes word line 607).

FIG. 6Q depicts one embodiment of an adjustable resistance bit linestructure. The adjustable resistance bit line structure may connect to aglobal bit line, such as global bit line 616, at the bottom of theadjustable resistance bit line structure. As depicted, the adjustableresistance bit line structure includes a select gate 618 and adielectric layer 610 arranged between the select gate 618 and anadjustable resistance local bit line 604. The adjustable resistancelocal bit line 604 may comprise intrinsic or undoped polysilicon. Insome cases, the adjustable resistance local bit line 604 may compriselightly doped N-polysilicon. The dielectric layer 610 may comprise anoxide layer. The select gate 618 may comprise TiN or polysilicon. An SGline 602 connects to a drain of a vertical TFT formed usingP-polysilicon layer 603. The source of the vertical TFT is connected tothe select gate 618. The vertical TFT may comprise an NMOS transistor.In some cases, the vertical TFT may comprise a JFET. The SG line 602 maycomprise TiN, polysilicon, or tungsten. In some cases, the SG line 602may extend in the word line direction (e.g., into the page). A ReRAMlayer 614 is arranged between the word lines 606-609 and the adjustableresistance local bit line 604. The ReRAM layer 614 may comprise a phasechange material, a ferroelectric material, or a metal oxide such asnickel oxide or hafnium oxide. The adjustable resistance local bit line604 may connect to a global bit line 616 via an integrated bottomresistor comprising an N+ polysilicon layer 652, an N-polysilicon layer653, and an N+ polysilicon layer 654. The global bit line 616 maycomprise TiN or tungsten. In one embodiment, the spacing between wordlines (e.g., the spacing between word lines 606 and 607) may comprise 20nm and the spacing between the top of the global bit line 616 and thetop of the N+ polysilicon layer 652 may comprise 150 nm, 200 nm, or 300nm.

As depicted in FIG. 6Q, the ReRAM layer 614 does not extend to theglobal bit line 616. Instead, an oxide layer not depicted may existbetween the ReRAM layer 614 and the global bit line 616. One benefit ofusing the adjustable resistance bit line structure depicted in FIG. 6Qis that the etch depth for etching a memory hole in which the adjustableresistance bit line structure may be formed may be reduced. Anotherbenefit of using the adjustable resistance bit line structure depictedin FIG. 6Q is that memory cell reliability may be improved as theintegrated resistor may reduce voltage stress across memory cellsconnected to the adjustable resistance local bit line.

In one embodiment, the height (or the thickness) of the N-polysiliconlayer 653 may be adjusted or set in order to provide a particularresistance value or to provide a particular voltage drop across theintegrated resistor during a memory operation (e.g., during aprogramming operation). The lowest word line layer that includes wordline 608 may comprise a dummy word line layer.

FIG. 6R depicts one embodiment of an adjustable resistance bit linestructure. The adjustable resistance bit line structure may connect to aglobal bit line, such as global bit line 628, near the top of theadjustable resistance bit line structure or above the highest word linelayer. One benefit of making the connection to the global bit line nearthe top of the adjustable resistance bit line structure is that theReRAM layer 614 may not need to be etched and the adjustable resistancebit line structure may be formed using a more reliable ReRAM material.

As depicted, the adjustable resistance bit line structure includes aselect gate 618 and a dielectric layer 610 arranged between the selectgate 618 and an adjustable resistance local bit line 604. The adjustableresistance local bit line 604 may comprise intrinsic or undopedpolysilicon. In some cases, the adjustable resistance local bit line 604may comprise lightly doped N-polysilicon. The dielectric layer 610 maycomprise an oxide layer. The select gate 618 may comprise TiN orpolysilicon. An SG line 602 connects to a drain of a vertical TFT formedusing P-polysilicon layer 603 and dielectric layer 610. The source ofthe vertical TFT is connected to the select gate 618. The vertical TFTmay comprise an NMOS transistor. In some cases, the vertical TFT formedusing P-polysilicon layer 603 may use a thinner dielectric layer thanthe dielectric layer 610 (e.g., a gate oxide for the vertical TFT may bethinner than the dielectric layer 610). The gate of the vertical TFT maybe controlled by gate control line 605 that extends in the global bitline direction. The gate control line 605 may comprise TiN or tungsten.A ReRAM layer 614 is arranged between the word lines 607-609 and theadjustable resistance local bit line 604. The ReRAM layer 614 maycomprise a phase change material, a ferroelectric material, or a metaloxide such as nickel oxide or hafnium oxide. The base layer 626 maycomprise a portion of a silicon substrate, a layer of polysilicon, or anoxide layer.

The adjustable resistance local bit line 604 may connect to the globalbit line 628 via an N+ polysilicon layer 624. The N+ polysilicon layer624 may be formed on top of the adjustable resistance local bit line 604or positioned above the highest word line layer. The global bit line 628may comprise TiN or tungsten. Thus, an adjustable resistance local bitline (e.g., a vertical bit line that comprises undoped polysilicon) mayconnect to a global bit line that is positioned below the adjustableresistance local bit line or may connect to a global bit line that ispositioned above the adjustable resistance local bit line.

FIG. 6S depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 6R. In one example, the cross-sectional view takenalong line X-X of FIG. 6R may comprise a horizontal slice taken througha global bit line layer that includes global bit line 628.

FIG. 6T depicts one embodiment of the adjustable resistance bit linestructure depicted in FIG. 6R using a cross-sectional view in the wordline direction. As depicted, the SG line 602 extends in the word linedirection along with word lines 607 and 608. The global bit line 628extends in the global bit line direction (e.g., into the page). The gatecontrol line 605 for controlling the gate of the vertical TFT may alsoextend in the global bit line direction.

FIG. 6U depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 6T. In one example, the cross-sectional view takenalong line X-X of FIG. 6T may comprise a horizontal slice taken througha word line layer that includes word line 607 and word line 609. Wordline 609 may comprise a word line that is in the same word line layer asword line 607 and adjacent to word line 607.

FIG. 6V depicts one embodiment of an adjustable resistance bit linestructure. The adjustable resistance bit line structure may connect to aglobal bit line, such as global bit line 628, near the top of theadjustable resistance bit line structure or above the highest word linelayer. One benefit of making the connection to the global bit line nearthe top of the adjustable resistance bit line structure is that theReRAM layer 614 may not need to be etched at the bottom of theadjustable resistance bit line structure (e.g., in order to connect anadjustable resistance local bit line to a global bit line located at thebottom of the adjustable resistance local bit line). As depicted, the SGline 602 directly connects to the select gate 618. Thus, in some cases,an SG line may directly connect to or abut a select gate. In othercases, an SG line may selectively connect to the select gate via avertical TFT or other transistor. One benefit of not using a verticalTFT to selectively connect the SG line to the select gate is thatprocess complexity and manufacturing costs may be reduced.

FIG. 6W depicts one embodiment of an adjustable resistance bit linestructure in which the adjustable resistance bit line structure isarranged in a horizontal orientation (e.g., positioned above a substrateand in a plane that is parallel to the substrate). The adjustableresistance bit line structure may connect to a global bit line, such asglobal bit line 672, at a first side of the adjustable resistance bitline structure. As depicted, the adjustable resistance bit linestructure includes a select gate 678 and a dielectric layer 679 arrangedbetween the select gate 678 and an adjustable resistance local bit line676. The adjustable resistance local bit line 676 may comprise intrinsicor undoped polysilicon. In some cases, the adjustable resistance localbit line 676 may comprise lightly doped N-polysilicon. The dielectriclayer 679 may comprise an oxide layer. The select gate 678 may compriseTiN or polysilicon. An SG line 677 connects to the select gate 678 on asecond side of the adjustable resistance bit line structure. The SG line677 may comprise TiN, polysilicon, or tungsten. A ReRAM layer 674 isarranged between the word lines including word line 675 and theadjustable resistance local bit line 676. The word lines including wordline 675 may comprise TiN, polysilicon, or tungsten (W). The ReRAM layer674 may comprise a phase change material, a ferroelectric material, or ametal oxide such as nickel oxide or hafnium oxide. The adjustableresistance local bit line 676 may connect to the global bit line 672 viaan N+ polysilicon region 673. The global bit line 672 may comprise TiNor tungsten. In some cases, a vertical global bit line, such as globalbit line 672, may connect to a plurality of adjustable resistance localbit lines that are arranged in a plurality of horizontal planes locatedabove a substrate. In some cases, a vertical SG line may connect to aplurality of select gates that are arranged in a plurality of horizontalplanes located above a substrate. In other cases, a horizontal SG linemay connect to a plurality of select gates that are arranged within ahorizontal plane located above a substrate.

FIG. 7A depicts one embodiment of a top plan view of an adjustableresistance bit line structure. The adjustable resistance bit linestructure may be formed using a vertical pillar that includes a selectgate SG 706 that is surrounded by an oxide layer Oxide 708 that issurrounded by an adjustable resistance local bit line AR_LBL 710 that issurrounded by a memory element layer 712. A first side of the verticalpillar may contact a first word line WL 702 and a second side of thevertical pillar may contact a second word line WL 703. In some cases,the word line spacing between word line WL 702 and word line WL 703 maycomprise 48 nm or 24 nm.

FIG. 7B depicts another embodiment of a top plan view of an adjustableresistance bit line structure. The adjustable resistance bit linestructure may be formed using a pair of vertical pillars. The firstvertical pillar of the pair of vertical pillars includes a select gate729 surrounded by an oxide layer 728 and the second vertical pillar ofthe pair of vertical pillars includes an adjustable resistance local bitline 726 surrounded by a memory element layer 724. The select gate 729may comprise TiN or polysilicon. The oxide layer 728 may comprisesilicon dioxide. The adjustable resistance local bit line 726 maycomprise undoped polysilicon or lightly doped N-polysilicon. The memoryelement layer 724 may comprise a phase change material, a ferroelectricmaterial, or a metal oxide such as nickel oxide or hafnium oxide. Afirst side of the second vertical pillar contacts a word line 722. Asdepicted, a third vertical pillar that includes select gate 723 may bepositioned such that the third vertical pillar and the first verticalpillar abut the second vertical pillar. In this case, the resistance orthe conductivity of the adjustable resistance local bit line 726 may beadjusted via an application of a voltage to select gate 729, select gate723, or both select gates 729 and 723 at the same time. In one example,the adjustable resistance local bit line 726 may be set into either aconducting state or a non-conducting state based on the voltages appliedto select gates 729 and 723.

FIG. 7C depicts another embodiment of a top plan view of an adjustableresistance bit line structure. The adjustable resistance bit linestructure may be formed using a pair of vertical pillars. The firstvertical pillar of the pair of vertical pillars includes a select gatepillar 739 and the second vertical pillar of the pair of verticalpillars includes an adjustable resistance local bit line pillar 736. Theselect gate pillar 739 and the adjustable resistance local bit linepillar 736 are separated by a dielectric layer 738. As depicted, anotherselect gate pillar 733 may be positioned such that the adjustableresistance local bit line pillar 736 is located between the select gatepillar 733 and the select gate pillar 739. The select gate pillar 739may comprise TiN or polysilicon. The dielectric layer 738 may comprisesilicon dioxide. The adjustable resistance local bit line pillar 736 maycomprise undoped polysilicon or lightly doped N-polysilicon. The memoryelement layer 734 may comprise a phase change material, a ferroelectricmaterial, or a metal oxide such as nickel oxide or hafnium oxide. Thememory element layer 734 may be positioned between the adjustableresistance local bit line pillar 736 and the word line 732. In somecases, the resistance or the conductivity of at least a portion of theadjustable resistance local bit line pillar 736 may be adjusted via anapplication of a voltage to the select gate pillar 739, the select gatepillar 733, or both select gate pillars 739 and 733 at the same time. Inone embodiment, the adjustable resistance local bit line pillar 736 maybe set into either a conducting state or a non-conducting state based onthe voltages applied to select gate pillars 739 and 733.

FIG. 7D depicts one embodiment of a cross-sectional view taken alongline Z-Z of FIG. 7C. As depicted, the adjustable resistance local bitline pillar 736 connects to a global bit line GBL 744 via N+ polysiliconlayer 742 and the select gate pillar 739 connects to a source of avertical TFT that is formed using P-polysilicon layer 752. The sourceand drain junctions of the vertical TFT comprise N+ polysilicon layer745 and N+ polysilicon layer 746. The drain of the vertical TFT thatincludes P-polysilicon layer 752 connects to the SG line SGL 758 thatruns in the word line direction. The vertical TFT may comprise an NMOStransistor. In some cases, the vertical TFT formed using P-polysiliconlayer 752 may use a thinner dielectric layer than the dielectric layer738 (e.g., a gate oxide for the vertical TFT may be thinner than thedielectric layer 738 arranged between the select gate pillar SG 739 andthe adjustable resistance local bit line pillar AR_LBL 736). Theadjustable resistance local bit line pillar 739 may comprise lightlydoped N-polysilicon. The SG line SGL 758 may comprise TiN, polysilicon,or tungsten. The gate of the vertical TFT that includes P-polysiliconlayer 752 may be controlled by control line CSG 754. The control lineCSG 754 may comprise TiN, polysilicon, or tungsten. As depicted, theselect gate pillar 733 connects to a source of a vertical TFT that isformed using P-polysilicon layer 753. The drain of the vertical TFT thatincludes P-polysilicon layer 753 connects to the SG line SGL 758. Boththe control line CSG 754 and the control line CSG 756 may be arranged inthe global bit line direction that is orthogonal to the word linedirection. As P-polysilicon layer 753 is arranged between the controlline CSG 754 and the control line CSG 756, the voltages applied to thecontrol line CSG 754 and the control line CSG 756 may be used to set thevertical TFT that includes P-polysilicon layer 753 into a conductingstate or a non-conducting state. In some cases, a voltage applied to thecontrol line CSG 754 may be used to set both the vertical TFT thatincludes P-polysilicon layer 752 and the vertical TFT that includesP-polysilicon layer 753 into conducting or non-conducting states.

In one embodiment, an integrated bottom resistor, such as the integratedbottom resistor depicted in FIG. 6H, may be placed in series between theadjustable resistance local bit line pillar 736 and the global bit lineGBL 744.

FIG. 7E depicts one embodiment of a side view of the adjustableresistance bit line structure depicted in FIG. 7C. As depicted, the SGline SGL 758 extends in the word line direction along with word linesthat include word line WL 732. The global bit line GBL 744 extends inthe global bit line direction (e.g., into the page). The control linesCSG 754 and 756 for controlling the gates of vertical TFTs may alsoextend in the global bit line direction.

FIG. 7F depicts one embodiment of the adjustable resistance bit linestructure depicted in FIG. 7E using a cross-sectional view in the globalbit line direction. As depicted, the SG line SGL 758 extends in the wordline direction (e.g., into the page) along with word line 732. The wordline layers may be separated from each other using an oxide layer, suchas oxide layer 761. An SG line SGL 759 may comprise an SG line that isin the same SG line layer as SG line SGL 758 and adjacent to SG line SGL758. The control line CSG 754 for controlling the gates of vertical TFTsand the global bit line GBL 744 extend in the global bit line direction(e.g., horizontally from left to right).

FIG. 7G depicts one embodiment of a side view of the adjustableresistance bit line structure depicted in FIG. 7C. As depicted, the SGline SGL 758 extends in the word line direction along with word linesthat include word line WL 732. The global bit line GBL 744 extends inthe global bit line direction (e.g., into the page). The control lineCSG 755 for controlling a gate of a vertical TFT formed usingP-polysilicon layer 752 may also extend in the global bit linedirection. The control line CSG 755 may comprise TiN, polysilicon, ortungsten. In this case, the control line CSG 755 may control the gate ofthe vertical TFT connecting the SG line SGL 758 to the select gatepillar SG 739. Each vertical TFT positioned above each select gate mayinclude a wrap-around gate. The wrap-around gates for each vertical TFTmay be separated from each other using a dielectric layer or an oxidelayer not depicted. As depicted, the source and drain junctions of thevertical TFT formed using P-polysilicon layer 752 correspond with N+polysilicon layer 745 and N+ polysilicon layer 746. The drain of thevertical TFT that includes P-polysilicon layer 752 connects to the SGline SGL 758 that runs in the word line direction. The vertical TFT maycomprise an NMOS transistor. In some cases, the vertical TFT formedusing P-polysilicon layer 752 may use a thinner dielectric layer thanthe dielectric layer 738. For example, the gate oxide 731 for thevertical TFT may be thinner than the dielectric layer 738 arrangedbetween the select gate pillar SG 739 and the adjustable resistancelocal bit line pillar AR_LBL 736.

FIG. 7H depicts one embodiment of a side view of the adjustableresistance bit line structure depicted in FIG. 7C. As depicted, the SGline SGL 758 extends in the word line direction along with word linesthat include word line WL 732. The global bit line GBL 744 extends inthe global bit line direction (e.g., into the page). The SG line SGL 758directly connects to the select gate pillar SG 739 (i.e., no verticalTFT is used to selectively connect the SG line SGL 758 to the selectgate pillar SG 739). Thus, in some cases, an SG line may directlyconnect to or abut a select gate. In other cases, an SG line mayselectively connect to a select gate via a vertical TFT or othertransistor. One benefit of not using a vertical TFT to selectivelyconnect the SG line to a select gate is that process complexity andmanufacturing costs may be reduced.

FIG. 7I depicts one embodiment of a perspective view of a portion of theadjustable resistance bit line structure depicted in FIG. 7C. Asdepicted, the select gate SG 733 and the adjustable resistance local bitline AR_LBL 736 comprise vertical pillar structures. The SG line SGL 758extends in the word line direction along with word line WL 732. The SGline SGL 758 may contact the select gate SG 733 directly or connect tothe select gate SG 733 via an N+ polysilicon layer not depicted. In someembodiments, the SG line SGL 758 may selectively connect to the selectgate SG 733 via a vertical TFT. The adjustable resistance local bit lineAR_LBL 736 may connect to the global bit line GBL 744 via an N+polysilicon layer not depicted. In some embodiments, the adjustableresistance local bit line AR_LBL 736 may selectively connect to theglobal bit line GBL 744 via a vertical TFT.

FIG. 7J depicts one embodiment of a perspective view of a portion of theadjustable resistance bit line structure depicted in FIG. 7A. Asdepicted, a single vertical pillar structure may include the select gateSG 706 and the adjustable resistance local bit line AR_LBL 710surrounding the select gate SG 706. The select gate SG 706 may extendthrough the center of the vertical pillar and may be isolated from orseparated from the adjustable resistance local bit line AR_LBL 710 by anoxide layer or a dielectric layer not depicted. The SG line SGL 788extends in the word line direction along with word line WL 702. The SGline SGL 788 may contact the select gate SG 706 directly or connect tothe select gate SG 706 via an N+ polysilicon layer not depicted. In someembodiments, the SG line SGL 788 may selectively connect to the selectgate SG 706 via a vertical TFT. The adjustable resistance local bit lineAR_LBL 710 may connect to the global bit line GBL 784 via an N+polysilicon layer not depicted. In some embodiments, the adjustableresistance local bit line AR_LBL 710 may selectively connect to theglobal bit line GBL 784 via a vertical TFT.

FIG. 8A depicts one embodiment of a top plan view of a portion of amemory array that includes select gate lines for controlling adjustableresistance bit line structures. As depicted, the portion of the memoryarray includes word lines, such as word line WL 292 and word line WL293, adjustable resistance local bit lines, such as adjustableresistance local bit line AR_LBL 291, and SG lines for controlling theadjustable resistance local bit lines, such as SG lines SGL 295 and SGL296. Outside the memory array, word line drivers, such as word linedriver WL driver 297, may connect to and drive the word lines of thememory array. In some cases, the word line drivers may be arranged suchthat odd numbered word lines are driven from one side of the memoryarray and even numbered word lines are driven from the other side of thememory array. In one embodiment, if a memory cell corresponding withReRAM 294 comprises a selected memory cell during a memory operation,then the word line WL 292 connected to the memory cell may be set to aselected word line voltage during the memory operation and theadjustable resistance local bit line AR_LBL 291 connected to the memorycell may be set into a conducting state during the memory operation. Theadjustable resistance local bit line AR_LBL 291 connected to the memorycell may be set into the conducting state by applying a selected gateline voltage to the SG line SGL 295 during the memory operation. Theadjustable resistance local bit lines connected to the SG line SGL 296may be set into non-conducting states during the memory operation byapplying an unselected gate line voltage to the SG line SLG 296 duringthe memory operation. The memory operation may comprise, for example, aread operation, a write operation, a programming operation, a programverify operation, or an erase verify operation.

FIG. 8B depicts an alternative embodiment of a top plan view of aportion of a memory array that includes select gate lines forcontrolling adjustable resistance bit line structures. As depicted, theportion of the memory array includes word lines, such as word line WL892 and word line WL 893, adjustable resistance local bit lines, such asadjustable resistance local bit line AR_LBL 891, and SG lines forcontrolling the adjustable resistance local bit lines, such as SG linesSGL 895 and SGL 896. The word line 892 and the word line 893 maycomprise word line comb structures. A word line comb structure maycomprise a plurality of word line segments (or fingers) within a wordline layer that are shorted together. For example, a first word linesegment of word line WL 892 connects to a first memory cellcorresponding with ReRAM 894 and a second word line segment of word lineWL 892 connects to a second memory cell corresponding with ReRAM 898. Inanother example, a word line comb structure may include a plurality offingers (e.g., 16 fingers or 32 fingers) that are shorted togetherwithin a single word line layer (e.g., one of 16 word line layers formedabove a substrate). In some cases, a first word line comb structurewithin a first word line layer may be positioned above a second wordline comb structure within a second word line layer positioned above thefirst word line layer.

As depicted in FIG. 8B, the word line comb associated with word line WL892 may be interdigitated with the word line comb associated with wordline WL 893. One benefit of using word line comb structures is that thearea constraints and/or pitch constraints for implementing word linedrivers for driving the word line comb structures may be reduced. Forexample, in FIG. 8A, word line drivers on one side of the memory arraymay have to drive word lines every two word line pitches (e.g., all evenword lines are driven from one side of the memory array). In contrast, asingle word line driver may drive the word line comb structureassociated with word line WL 892. In one embodiment, a word line combstructure may comprise two or more word line segments within a word linelayer that are shorted together. The two or more word line segments mayspan a memory array and be shorted together within the word line layerat an end of the memory array. The word line comb structure may bearranged in a horizontal plane above a substrate. In another embodiment,a word line comb structure may comprise two or more word line segmentswithin a word line layer that span at least a portion of a memory arrayand that are shorted together within the word line layer at an end ofthe memory array.

In one embodiment, a first word line comb may include a plurality ofword line segments. The plurality of word line segments may include afirst word line segment and a second word line segment. The first wordline segment may connect to a first memory cell and the second word linesegment may connect to a second memory cell. The first memory cell mayconnect to a first adjustable resistance bit line structure and thesecond memory cell may connect to a second adjustable resistance bitline structure. A first SG line may control a select gate of the firstadjustable resistance bit line structure and a second SG line maycontrol a select gate of the second adjustable resistance bit linestructure. In one example, the first adjustable resistance bit linestructure may be set into a conducting state and the second adjustableresistance bit line structure may be set into a non-conducting stateduring a memory operation. In another example, the first adjustableresistance bit line structure may be set into a non-conducting state andthe second adjustable resistance bit line structure may be set into aconducting state during a memory operation. Thus, although both thefirst word line segment and the second word line segment may be set to aselected word line voltage during a memory operation, the firstadjustable resistance bit line structure and the second adjustableresistance bit line structure may be set into different conductingstates. For example, the adjustable resistance local bit line AR_LBL 891in FIG. 8B connected to the memory cell corresponding with ReRAM 894 maybe set into a conducting state via application of a selected SG linevoltage to SG line SGL 895 and the adjustable resistance local bit lineconnected to the memory cell corresponding with ReRAM 898 may be setinto a non-conducting state via application of an unselected SG linevoltage to SG line SGL 896.

The ability to selectively set adjustable resistance bit line structureswithin a memory array connected to memory cells that are connected to aselected word line comb into conducting states or non-conducting statesmay allow large word line comb structures to be utilized, which mayimprove memory array efficiency and reduce overall memory chip area.Furthermore, the ability to set non-selected adjustable resistance bitline structures that are connected to a selected word line comb intonon-conducting states allows for word line combs with an increasednumber of fingers and eliminates the need for vertical TFTs forselectively connecting global bit lines to the adjustable resistance bitline structures. In one example, a vertical TFT arranged between aglobal bit line and an adjustable resistance bit line structure may beunnecessary for selectivity reasons because the adjustable resistancebit line structure may be set into a non-conducting state viaapplication of an unselected select gate voltage to a select gate of theadjustable resistance bit line structure.

FIG. 8C depicts one embodiment of a top plan view of portions of twomemory arrays in which a plurality of select gate lines for controllingadjustable resistance bit line structures within the two memory arraysspan both memory arrays. As depicted, a first memory array of the twomemory arrays includes portions of word line WL 892 and word line WL893, adjustable resistance local bit line AR_LBL 891, and memory cellscorresponding with ReRAM 894 and ReRAM 898. The second memory array ofthe two memory arrays includes portions of word line WL 892 and wordline WL 899 and a memory cell corresponding with ReRAM 897. In thiscase, the SG lines SGL 895 and SGL 896 span both memory arrays. Thisallows the SG line drivers to be arranged at the ends or sides of bothmemory arrays rather than being arranged in the middle of or between thetwo memory arrays. In one example, a first set of SG line drivers may bearranged on a first side of the memory arrays and drive even numbered SGlines and a second set of SG line drivers may be arranged on a secondside of the memory arrays and drive odd numbered SG lines.

In some embodiments, the SG lines may span a plurality of memory arrays.In one example, the SG lines, such as SG line SGL 895 and SGL 896 mayspan 128 memory arrays or 512 memory arrays. In some embodiments, the SGlines may span a plurality of word line combs. In one example, the SGlines, such as SG line SGL 895 and SGL 896 may span 128 word line combsor 512 word line combs.

In one embodiment, a plurality of SG lines may span a plurality ofmemory arrays. During a memory operation, a first SG line of theplurality of SG lines may be selected causing the adjustable resistancelocal bit lines across all of the plurality of memory arrays connectedto the first SG line to be set into a conducting state. The other SGlines of the plurality of SG lines may be unselected causing theadjustable resistance local bit lines across all of the plurality ofmemory arrays connected to the other SG lines to be set into anon-conducting state. In some cases, a subset of the plurality of SGlines may be selected (e.g., two of the SG lines) while the other SGlines of the plurality of SG lines may be unselected. In some cases, SGline comb structures may be used in which a plurality of SG linesegments within an SG line layer are shorted together. For example, afirst SG line segment and a second SG line segment may be shortedtogether at the ends of a plurality of memory arrays which are spannedby the first SG line segment and the second SG line segment. The memoryoperation may comprise, for example, a read operation, a writeoperation, a programming operation, a program verify operation, or anerase verify operation. During the memory operation, memory cells withinone or more of the plurality of memory arrays may be read or programmedat the same time.

FIG. 8D depicts one embodiment of a portion of a memory array. Asdepicted, the memory array includes word lines WL0-WL63 and fouradjustable resistance bit line structures corresponding with selectgates SG 832, SG 834, SG 836, and SG 838. The adjustable resistance bitline structure corresponding with select gate SG 832 includes anadjustable resistance local bit line AR_LBL 812. The adjustableresistance bit line structure corresponding with select gate SG 834includes an adjustable resistance local bit line AR_LBL 814. Theadjustable resistance bit line structure corresponding with select gateSG 836 includes an adjustable resistance local bit line AR_LBL 816. Theadjustable resistance bit line structure corresponding with select gateSG 838 includes an adjustable resistance local bit line AR_LBL 818. Afirst global bit line GBL 802 connects to the adjustable resistancelocal bit lines AR_LBL 812 and AR_LBL 814. A second global bit line GBL804 connects to the adjustable resistance local bit lines AR_LBL 816 andAR_LBL 818. In some cases, the global bit lines GBL 802 and GBL 804 mayconnect to the adjustable resistance local bit lines at the bottom ofthe adjustable resistance bit line structures. In other cases, theglobal bit lines GBL 802 and GBL 804 may connect to the adjustableresistance local bit lines at the top of the adjustable resistance bitline structures. As depicted, the word lines WL0-WL31 connect to theadjustable resistance bit line structure corresponding with select gateSG 832 and the adjustable resistance bit line structure correspondingwith select gate SG 836. The word lines WL32-WL63 connect to theadjustable resistance bit line structure corresponding with select gateSG 834 and the adjustable resistance bit line structure correspondingwith select gate SG 838.

In one embodiment, the select gates SG 832, SG 834, SG 836, and SG 838may be set to the same voltage or to different voltages. In one example,select gate SG 832 may be set to a first voltage, select gate SG 834 maybe set to a second voltage, select gate SG 836 may be set to a thirdvoltage, and select gate SG 838 may be set to a fourth voltage. Inanother embodiment, each of the select gates SG 832, SG 834, SG 836, andSG 838 may be set to a selected select gate voltage (e.g., 4V) or to anunselected select gate voltage (e.g., 0V) independent of the biasingconditions applied to the other select gates. In one example, theadjustable resistance bit line structure corresponding with select gateSG 836 may be set into a conducting state while the adjustableresistance bit line structures corresponding with select gates SG 832,SG 834, and SG 838 are set into non-conducting states.

In another embodiment, select gates SG 832 and SG 836 may be set to afirst voltage and select gates SG 834 and SG 838 may be set to a secondvoltage different from the first voltage. In this case, the select gatesSG 832 and SG 836 may be connected to a first SG line and the selectgates SG 834 and SG 838 may be connected to a second SG line. In oneexample, the adjustable resistance bit line structures correspondingwith select gates SG 832 and SG 836 may be set into conducting stateswhile the adjustable resistance bit line structures corresponding withselect gates SG 834 and SG 838 may be set into non-conducting states. Inanother embodiment, select gates SG 832 and SG 834 may be set to a firstvoltage and select gates SG 836 and SG 838 may be set to a secondvoltage different from the first voltage. In this case, the select gatesSG 832 and SG 834 may be connected to a first SG line and the selectgates SG 836 and SG 838 may be connected to a second SG line. In oneexample, the adjustable resistance bit line structures correspondingwith select gates SG 832 and SG 834 may be set into conducting stateswhile the adjustable resistance bit line structures corresponding withselect gates SG 836 and SG 838 may be set into non-conducting states.

FIG. 8E depicts one embodiment of the portion of a memory array depictedin FIG. 8D during a memory operation. As depicted, the memory arrayincludes word lines WL0-WL63 and four adjustable resistance bit linestructures corresponding with select gates SG 832, SG 834, SG 836, andSG 838. During the memory operation, the adjustable resistance bit linestructures corresponding with select gates SG 832 and SG 836 have beenset into conducting states while the adjustable resistance bit linestructures corresponding with select gates SG 834 and SG 838 have beenset into non-conducting states. In some cases, a first SG line notdepicted may directly connect to both the select gate 832 and the selectgate 836 (e.g., the first SG line may extend in the word linedirection). Selected word line WL15 has been set to a selected word linevoltage and the selected global bit line GBL 804 has been set to aselected global bit line voltage. Unselected word lines WL0-14 andWL16-63 have been set to unselected word line voltages and unselectedglobal bit line GBL 802 has been set to an unselected global bit linevoltage. Under these biasing conditions, the memory cell 851 connectedto word line WL15 and the adjustable resistance local bit line AR_LBL816 may comprise an S cell, the memory cells 852-854 connected to theadjustable resistance local bit line AR_LBL 816 may comprise F cells,the memory cell 841 connected to word line WL15 and the adjustableresistance local bit line AR_LBL 812 may comprise an H cell, the memorycells 842-844 connected to the adjustable resistance local bit lineAR_LBL 812 may comprise U cells, the memory cells 855-858 connected tothe adjustable resistance local bit line AR_LBL 818 may comprise Ucells, and the memory cells 845-848 connected to the adjustableresistance local bit line AR_LBL 814 may comprise U cells.

In one embodiment, during a read operation, the selected word linevoltage may be 0V, the selected global bit line voltage may be 3V, theunselected word line voltage may be 3V, and the unselected global bitline voltage may be 0V. In one embodiment, during a RESET operation, theselected word line voltage may be 0V, the selected global bit linevoltage may be 5V, the unselected word line voltage may be 3V, and theunselected global bit line voltage may be 0V. In one embodiment, duringa SET operation, the selected word line voltage may be 5V, the selectedglobal bit line voltage may be 0V, the unselected word line voltage maybe 0V, and the unselected global bit line voltage may be 5V.

In some cases, the select gates SG 832 and SG 836 may be set to aselected select gate voltage during the memory operation and the selectgates SG 834 and SG 838 may be set to an unselected select gate voltage.In one example, during a read operation, the selected select gatevoltage may be 5V and the unselected select gate voltage may be 0V. Inanother example, during a RESET operation, the selected select gatevoltage may be 7V and the unselected select gate voltage may be 0V. Inanother example, during a SET operation, the selected select gatevoltage may be 5V and the unselected select gate voltage may be 0V. Theselected select gate voltage used during a RESET operation may bedifferent from the selected select gate voltage used during a SEToperation. The selected select gate voltage used during a RESEToperation may be greater than the selected select gate voltage usedduring a SET operation.

In some embodiments, the selected select gate voltage applied to aselect gate during a programming operation may be determined based on amaximum current limit through a selected memory cell. The maximumcurrent limit through the selected memory cell may depend on whether theprogramming operation comprises a SET operation (e.g., a maximum currentlimit of 10 μA or a maximum current limit of 50 μA) or a RESET operation(e.g., a maximum current limit of 50 μA or a maximum current limit of150 μA). In one example, the selected select gate voltage may be used toprovide current limiting in order to prevent over-programming of theselected memory cell during the programming operation. In some cases, amemory array may include a first select gate line connected to a firstselect gate of a first adjustable resistance bit line structure and asecond select gate line connected to a second select gate of a secondadjustable resistance bit line structure. In another example, during amemory operation, the first select gate line may be set to an unselectedselect gate voltage (e.g., 0V) and the second select gate line may beset to a current limiting voltage (e.g., 4V) in order to limit themaximum current through a selected memory cell connected to the secondadjustable resistance bit line structure (e.g., to limit the currentthrough the selected memory cell to be at most 10 μA). In anotherexample, during a memory operation, the first select gate line may beset to a first current limiting voltage (e.g., 3.5V) in order to limitthe maximum current through a selected memory cell connected to thefirst adjustable resistance bit line structure (e.g., to limit thecurrent through the selected memory cell to be at most 5 μA) and thesecond select gate line may be set to an unselected select gate voltage(e.g., 0V).

FIG. 8F depicts one embodiment of the portion of the memory arraydepicted in FIG. 8D during a memory operation. As depicted, the memoryarray includes word lines WL0-WL15 and two adjustable resistance bitline structures corresponding with select gates SG 832 and SG 836. Theselect gates SG 832 and SG 836 are shorted together via a select gateline SGL 803. During the memory operation, the select gate line SGL 803is biased to 7V, the selected word line WL15 is biased to 0V, theunselected word line WL0 is biased to 1.33V, the selected global bitline GBL 804 is biased to 2V, and the unselected global bit line GBL 802is biased to 0.66V. In this case, the voltage across the S cell is 2.0V(i.e., 2.0V-0V), the voltage across the F cell is 0.66V (i.e.,2V-1.33V), the voltage across the U cell is 0.66V (i.e., 1.33V-0.66V),and the voltage across the H cell is 0.66V (i.e., 0.66V-0V). Thus, thevoltage stress across the F cell, the U cell, and the H cell may be thesame or substantially the same. In one example, the voltage stressacross the F cell, the U cell, and the H cell may be set to one third ofthe voltage across the S cell (e.g., if 2V is applied across an S cell,then one third of the 2V across the S cell is roughly 0.66V). In somecases, the memory operation may comprise a read operation or a RESEToperation.

In some embodiments, a selected global bit line may be set to a voltagethat is greater than the voltage applied at the channel locationassociated with the lowest F cell (e.g., the channel location associatedwith WL0 in FIG. 8F) or set to a voltage that is greater than thevoltage applied to a channel location associated with the F cell closestto the selected global bit line. In one example, the selected global bitline may be biased to 4V and the channel location associated with the Fcell closest to the selected global bit line may be biased to 2V due toIR drop between the selected global bit line and the channel locationassociated with the lowest F cell.

In one embodiment, during a read operation, the voltage differenceacross F cells may be minimized or set to 0V in order to minimize theimpact of leakage currents from affecting the sensing of a selectedmemory cell. In one example, the voltages applied to unselected wordlines, such as unselected word line WL0, may be determined based onvoltage drops occurring along an adjustable resistance local bit line.

FIG. 8G depicts another embodiment of the portion of the memory arraydepicted in FIG. 8D during a memory operation. As depicted, the memoryarray includes word lines WL0-WL15 and two adjustable resistance bitline structures corresponding with select gates SG 832 and SG 836. Theselect gates SG 832 and SG 836 are shorted together via a select gateline SGL 803. During the memory operation, the select gate line SGL 803is biased to 5V, the selected word line WL15 is biased to 2V, theunselected word line WL0 is biased to 0.66V, the selected global bitline GBL 804 is biased to 0V, and the unselected global bit line GBL 802is biased to 1.33V. In this case, the voltage across the S cell is 2.0V(i.e., 2.0V-0V), the voltage across the F cell is 0.66V (i.e.,0.66V-0V), the voltage across the U cell is 0.66V (i.e., 1.33V-0.66V),and the voltage across the H cell is 0.66V (i.e., 2V-1.33V). Thus, thevoltage stress across the F cell, the U cell, and the H cell may be thesame or substantially the same. In one example, the voltage stressacross the F cell, the U cell, and the H cell may be set to one third ofthe voltage across the S cell (e.g., if 2V is applied across an S cell,then one third of the 2V across the S cell is roughly 0.66V). In somecases, the memory operation may comprise a SET operation.

FIG. 8H depicts a flowchart describing one embodiment of a process forperforming a memory operation. In one embodiment, the process of FIG. 8Hmay be performed by a memory system, such as memory system 101 in FIG.1A.

In step 862, a first word line within a memory array is identified. Thefirst word line may correspond with a word line to be selected during amemory operation. In step 863, a first global bit line within the memoryarray is identified. The first global bit line may correspond with aglobal bit line to be selected during a memory operation. The first wordline and the first global bit line may be identified based on a memoryinstruction and/or a memory address. The first global bit line may beconnected to an adjustable resistance bit line structure. The adjustableresistance bit line structure may include an adjustable resistance localbit line and a select gate. A first memory cell may be arranged betweenthe adjustable resistance local bit line and the first word line. Instep 864, a second global bit line within the memory array isidentified. The second global bit line may be connected to a secondadjustable resistance bit line structure different from the firstadjustable resistance bit line structure. The second adjustableresistance bit line structure may include a second adjustable resistancelocal bit line and a second select gate. A second memory cell may bearranged between the second adjustable resistance local bit line and thefirst word line.

In step 865, the adjustable resistance local bit line is set into aconducting state by applying a first voltage to the select gate. In oneexample, the first voltage may comprise 5V or 7V. In step 866, thesecond adjustable resistance local bit line is set into a non-conductingstate by applying a second voltage different from the first voltage tothe second select gate. In one example, the second voltage may comprise0V. In some cases, the second adjustable resistance local bit line maybe set into a highly resistive state (e.g., more than 1 Gohm or morethan 10 Gohm) by applying 0V to the second select gate. In someembodiments, the setting of the second adjustable resistance local bitline into the non-conducting state may be performed prior to the settingof the adjustable resistance local bit line into the conducting state.In some embodiments, a plurality of adjustable resistance local bitlines may be set into non-conducting states prior to setting one or moreadjustable resistance local bit lines of the plurality of adjustableresistance local bit lines into conducting states. The one or moreadjustable resistance local bit lines may correspond with adjustableresistance local bit lines that are connected to a word line to beselected during the memory operation. In step 867, a memory operation isperformed on the memory array. The memory operation may include applyinga selected word line voltage to the first word line and a selected bitline voltage to the first global bit line while the adjustableresistance local bit line is set into a conducting state and the secondadjustable resistance local bit line a set into the non-conductingstate. The memory operation may comprise a read operation, a writeoperation, a programming operation, a SET operation, a RESET operation,an erase operation, a program verify operation, or an erase verifyoperation.

In some embodiments, the setting the second adjustable resistance localbit line into the non-conducting state may comprise precharging thesecond select gate to the second voltage prior to setting the adjustableresistance local bit line into the conducting state by applying thefirst voltage to the select gate. In one example, a first vertical TFTmay selectively connect the select gate to a select gate line and asecond vertical TFT may selectively connect the second select gate tothe select gate line. In this case, the second select gate may beprecharged to the second voltage by setting the second vertical TFT intoa conducting state while driving the select gate line to the secondvoltage. After the second select gate has been precharged to the secondvoltage, then the select gate may be biased to the first voltage bysetting the second vertical TFT into a non-conducting state and thefirst vertical TFT into a conducting state while driving the select gateline to the first voltage.

In some embodiments, a select gate line may directly connect to theselect gate and the second select gate (e.g., a thin-film transistor maynot be positioned between the select gate line and the second selectgate). In this case, during the memory operation, both the adjustableresistance local bit line and the second adjustable resistance local bitline may be set into conducting states. The memory array may include athird adjustable resistance bit line structure connected to the secondglobal bit line. The third adjustable resistance bit line structure mayinclude a third adjustable resistance local bit line and a third selectgate. A third memory cell may be arranged between the third adjustableresistance local bit line and a second word line of the memory array(e.g., an unselected word line). In this case, while both the adjustableresistance local bit line and the second adjustable resistance local bitline are set into a conducting state, the third adjustable resistancelocal bit line may be set into a non-conducting state (e.g., by applying0V to the third select gate).

In one embodiment, the first global bit line may be connected to afourth adjustable resistance bit line structure. The fourth adjustableresistance bit line structure may include a fourth adjustable resistancelocal bit line and a fourth select gate. A fourth memory cell may bearranged between the fourth adjustable resistance local bit line and asecond word line (e.g., an unselected word line). In this case, whileboth the adjustable resistance local bit line and the second adjustableresistance local bit line are set into a conducting state, the fourthadjustable resistance local bit line may be set into a non-conductingstate (e.g., by applying 0V to the fourth select gate).

FIG. 8I depicts a flowchart describing one embodiment of a process forperforming a read operation. In one embodiment, the process of FIG. 8Imay be performed by a memory system, such as memory system 101 in FIG.1A.

In step 872, a read command is received. In step 873, a first word lineand a first global bit line are determined based on the read command.The first global bit line may be connected to an adjustable resistancebit line structure. The adjustable resistance bit line structure mayinclude an adjustable resistance local bit line and a select gate. Afirst memory cell may be arranged between the adjustable resistancelocal bit line and the first word line. In step 874, a first select gateline is determined. The first select gate line may be connected to theselect gate. In step 875, the first select gate line is set to aselected select gate voltage (e.g., 4V). In step 876, the first wordline is set to a selected word line voltage (e.g., 0V). In step 877, thefirst global bit line is set to a selected bit line voltage (e.g., 3V).In step 878, a read operation is performed in response to receiving theread command. The read operation may include determining a first stateof the first memory cell while the first select gate line is set to theselected select gate voltage, the first word line is set to the selectedword line voltage, and the first global bit line is set to the selectedbit line voltage. In this case, setting the first select gate lineconnected to the select gate to the selected select gate voltage maycause the adjustable resistance local bit line to be set into aconducting state during the read operation. In one example, the firststate of the first memory cell may be determined using sensing circuitry(e.g., a sense amplifier) or a current comparison circuit that comparesthe current through the first memory cell with a reference current. Instep 879, data associated with the first state of the first memory cellis outputted.

FIG. 8J depicts a flowchart describing one embodiment of a process forperforming a programming operation. In one embodiment, the process ofFIG. 8J may be performed by a memory system, such as memory system 101in FIG. 1A.

In step 882, a programming command is received. In step 883, a firstword line and a first global bit line are determined based on theprogramming command. The first global bit line may be connected to anadjustable resistance bit line structure. The adjustable resistance bitline structure may include an adjustable resistance local bit line and aselect gate. A first memory cell may be arranged between (or disposedbetween) the adjustable resistance local bit line and the first wordline. In step 884, a first select gate line is determined. The firstselect gate line may be connected to the select gate. In step 885, aselected select gate voltage is determined. The selected select gatevoltage may be determined based on whether a SET or RESET operation isto be performed. If a SET operation is to be performed, then theselected select gate voltage may be set to 5V. If a RESET operation isto be performed, then the selected select gate voltage may be set to 7V.In step 886, a selected word line voltage is determined. The selectedword line voltage may be determined based on whether a SET or RESEToperation is to be performed. In step 897, a selected bit line voltageis determined. The selected bit line voltage may be determined based onwhether a SET or RESET operation is to be performed. In step 888, aprogramming operation is performed in response to receiving theprogramming command. The programming operation may include applying theselected select gate voltage to the first select gate line, applying theselected word line voltage to the first word line, and applying theselected bit line voltage to the first global bit line. In this case,setting the first select gate line connected to the select gate to theselected select gate voltage may cause the adjustable resistance localbit line to be set into a conducting state during the programmingoperation. In step 889, a verify operation is performed subsequent tothe programming operation to verify that the first memory cell wasprogrammed into the correct programming state.

In one embodiment, the programming operation may comprise a RESEToperation. In another embodiment, the programming operation may comprisea SET operation. In some cases, the selected select gate voltage appliedto the first select gate line during the programming operation may beset in order to limit the maximum current through the first memory cellbeing programmed during the programming operation. The selected selectgate voltage may be determined based on the type of programmingoperation to be performed. For example, if a RESET operation is to beperformed, then the selected select gate voltage may comprise a firstvoltage associated with a first current limiting value (e.g., 20 μA). Ifa SET operation is to be performed, then the selected select gatevoltage may comprise a second voltage less than the first voltageassociated with a second current limiting value (e.g., 10 μA).

FIG. 8K depicts one embodiment of an adjustable resistance bit linestructure during a RESET operation. As depicted, the selected word lineWL15 has been set to a selected word line voltage (e.g., 0V) and theselected global bit line GBL 804 has been set to a selected bit linevoltage (e.g., 5V). The select gate SG 836 may be set to a selectedselect gate voltage such that the adjustable resistance local bit lineAR_LBL 816 is set into a conducting state. However, although theadjustable resistance local bit line AR_LBL 816 is set into theconducting state, a channel resistance of a channel of the adjustableresistance local bit line AR_LBL 816 may cause voltage drops to occuralong the channel. As depicted, the channel voltage at the bottom of thechannel is 5V, the channel voltage at a channel location correspondingwith word line WL0 is 2.5V, and the channel voltage at a channellocation corresponding with word line WL15 is 1.5V. The voltage acrossthe selected memory cell may comprise the difference between theselected word line voltage and the channel voltage at the channellocation corresponding with word line WL15 (i.e., 1.5V). The slope 808of the channel voltage between the bottom of the channel and the channellocation corresponding with the bottommost word line may be set based onan unselected word line voltage applied to the word line WL0. In oneexample, if the unselected word line voltage applied to the word lineWL0 is 2.5V, then the channel voltage at the channel locationcorresponding with word line WL0 may be set to 2.5V or a voltage closeto 2.5V. In some cases, the voltage applied to the word line WL0 may beset in order to set the channel voltage at the channel locationcorresponding with word line WL0 to a particular channel voltage (e.g.,in order to set the channel voltage at the channel locationcorresponding with word line WL0 to 2V or to 3V). The slope 807 of thechannel voltage between the channel location corresponding with wordline WL15 and the channel location corresponding with word line WL0 mayalso be set based on the unselected word line voltage applied to theword line WL0 and the conductivity of the channel. The slope 807 mayalso be determined based on the selected select gate voltage applied tothe select gate SG 836. The slope 807 may also be determined based atemperature of a memory array or a memory chip. The slope 807 may alsobe determined based on the current through the selected memory cell orthe maximum current allowed through the selected memory cell (e.g., 100nA, 2 μA, or 50 μA).

In one embodiment, the voltage applied to the word line closest to theglobal bit line connection made at the bottom of the adjustableresistance bit line structure may be used to set or adjust the slope 807of the channel voltage between the channel location corresponding withthe selected word line (e.g., WL15 in FIG. 8K) and the channel locationcorresponding with the bottommost word line (e.g., WL0 in FIG. 8K). Thememory cell connected to the word line closest to the global bit linemay be left unformed or comprise a dummy memory cell. One benefit ofsetting the voltage applied to the word line closest to the global bitline connection to a voltage less than the selected bit line voltage isthat the voltage stress applied to unselected memory cells (e.g., Fcells) may be reduced.

In some embodiments, the adjustable resistance bit line structure andthe voltages applied to the word lines WL0-WL15 and the global bit lineGBL 804 depicted in FIG. 8K may be used during a read operation in whicha state of a selected memory cell connected to word line WL15 may bedetermined. In other embodiments, during a read operation, a selectedword line may be set to 0V, a selected global bit line may be set to 3V,the word line closest to the bottom of the adjustable resistance bitline structure may be set to 2V, and the other unselected word lines maybe set to 0.5V.

In some embodiments, the selected select gate voltage that is applied tothe select gate SG 836 may be determined based on the voltage applied tothe word line closest to the global bit line, the temperature of amemory array or a memory chip (e.g., sensed using a temperature sensorlocated on a memory chip), the maximum current limit setting for thecurrent allowed through the selected memory cell, and/or the type ofmemory operation to be performed (e.g., a read operation, a SEToperation, a RESET operation, or an erase operation).

FIG. 8L depicts one embodiment of word line voltage settings that may beused during the RESET operation depicted in FIG. 8K. As depicted, theselected word line WL15 has been set to 0V, the unselected word line WL0has been set to 2.5V in order to set the channel voltage at a channellocation corresponding with word line WL0 to be close to orsubstantially the same as 2.5V, and the other unselected word linesWL1-WL14 have been set to 1.75V. In this case, a voltage differenceacross the memory cell connected to word line WL1 may comprise 0.65V(i.e., 1.75V-2.4V). In some cases, this voltage difference may lead toprogram disturb of the memory cell connected to word line WL1.

FIG. 8M depicts another embodiment of word line voltage settings thatmay be used during the RESET operation depicted in FIG. 8K. As depicted,the selected word line WL15 has been set to 0V, the unselected word lineWL0 has been set to 2.5V in order to set the channel voltage at achannel location corresponding with word line WL0 to be close to orsubstantially the same as 2.5V, and the other unselected word linesWL1-WL14 have been set to voltages that correspond with the slope 807 ofthe channel voltage between the channel location corresponding with thetopmost word line (e.g., WL15 in FIG. 8K) and the channel locationcorresponding with the bottommost word line (e.g., WL0 in FIG. 8K). Insome cases, the voltages applied to the unselected word lines WL1-WL14may be set in order to minimize the voltage stress across each of thememory cells connected to the unselected word lines WL1-WL14. In oneexample, the voltages applied to the unselected word lines WL1-WL14 maybe set in order to place 0V or substantially close to 0V across each ofthe memory cells connected to the unselected word lines WL1-WL14.Setting the unselected word lines to location-dependent voltages thatminimize the voltage differences across unselected memory cells mayreduce program disturb and improve memory cell reliability. As depicted,the unselected word line voltages applied to the unselected word linesbetween word line WL15 and word line WL0 depend on the location of theunselected word line. For example, the unselected word line voltageapplied to word line WL1 may comprise 2.4V and the unselected word linevoltage applied to word line WL14 may comprise 1.6V.

In some embodiments, the voltages applied to unselected word linesduring a RESET operation, such as the unselected word lines WL1-WL14 inFIG. 8M, may be set in order to place at most a first voltage (e.g., atmost 500 mV or at most 1V) across each of the memory cells connected tothe unselected word lines. In other embodiments, the voltages applied toa first set of unselected word lines during a RESET operation may be setin order to place at most a first voltage (e.g., at most 500 mV or atmost 1V) across each of the memory cells connected to the first set ofunselected word lines. In one example, if a selected word line comprisesword line WL7, then the first set of word lines may comprise word linesWL0-WL6.

FIG. 8N depicts another embodiment of an adjustable resistance bit linestructure during a RESET operation. As depicted, a dummy word line WLDpositioned as the bottommost word line closest to the global bit lineconnection has been set to 2.6V, the selected word line WL15 has beenset to a selected word line voltage (e.g., 0V), and the selected globalbit line GBL 804 has been set to a selected bit line voltage (e.g., 5V).The dummy word line WLD may be set to 2.6V in order to set the channelvoltage at the channel location corresponding with word line WL0 to beclose to or substantially the same as 2.5V. The select gate SG 836 maybe set to a selected select gate voltage such that the adjustableresistance local bit line AR_LBL 816 is set into a conducting state.However, although the adjustable resistance local bit line AR_LBL 816 isset into the conducting state, a channel resistance of a channel of theadjustable resistance local bit line AR_LBL 816 may cause voltage dropsto occur along the channel. As depicted, the channel voltage at thebottom of the channel is 5V, the channel voltage at a channel locationcorresponding with dummy word line WLD is 2.6V, the channel voltage at achannel location corresponding with word line WL0 is 2.5V, and thechannel voltage at a channel location corresponding with word line WL15is 1.5V. The voltage across the selected memory cell may comprise thedifference between the selected word line voltage and the channelvoltage at the channel location corresponding with word line WL15 (i.e.,1.5V).

The slope 808 of the channel voltage between the bottom of the channeland the channel location corresponding with bottommost word line may beset based on an unselected word line voltage applied to the dummy wordline WLD. In one example, if the unselected word line voltage applied tothe dummy word line WLD is 2.6V, then the channel voltage at the channellocation corresponding with word line WLD may be set to 2.6V or to avoltage close to 2.6V. The slope 807 of the channel voltage between thechannel location corresponding with word line WL15 and the channellocation corresponding with dummy word line WLD may also be set based onthe unselected word line voltage applied to the dummy word line WLD andthe conductivity of the channel. The slope 807 may also be determinedbased on the selected select gate voltage applied to the select gate SG836. The slope 807 may also be determined based on the current throughthe selected memory cell (e.g., 100 nA, 2 μA, or 50 μA). In someembodiments, the slope 807 corresponding with the channel voltage alongthe channel between the channel location corresponding with word lineWL15 and the channel location corresponding with dummy word line WLD maybe adjusted based on the selected select gate voltage applied to theselect gate SG 836 (e.g., the slope may be increased by applying ahigher select gate voltage to the select gate SG 836; an increased slopecorresponds with a smaller voltage drop across the channel of anadjustable resistance local bit line).

In one embodiment, the voltage applied to the dummy word line WLD may beused to set or adjust the slope 807 of the channel voltage between thechannel location corresponding with the selected word line (e.g., wordline WL15 in FIG. 8N) and the channel location corresponding with thebottommost word line (e.g., word line WLD in FIG. 8N). The memory cellconnected to the dummy word line may be left unformed or set into a highresistance state. One benefit of setting the voltage applied to thedummy word line to a voltage less than the selected bit line voltage isthat the voltage stress applied to unselected memory cells may bereduced.

FIG. 8O depicts another embodiment of an adjustable resistance bit linestructure during a RESET operation. As depicted, a dummy word line WLDpositioned as the bottommost word line closest to the global bit lineconnection has been set to 2.6V, the selected word line WL15 has beenset to a selected word line voltage (e.g., 0V), and the selected globalbit line GBL 804 has been set to a selected bit line voltage (e.g.,4.5V). The select gate SG 836 may be set to a selected select gatevoltage such that the adjustable resistance local bit line AR_LBL 816 isset into a conducting state. As depicted, the channel voltage at thebottom of the channel is 4.5V, the channel voltage at a channel locationcorresponding with dummy word line WLD is 2.6V, the channel voltage at achannel location corresponding with word line WL0 is 2.5V, and thechannel voltage at a channel location corresponding with word line WL15is 1.5V. The slope 809 may represent an average channel voltage alongthe channel between the channel location corresponding with word lineWL15 and the channel location corresponding with dummy word line WLD.The channel voltage 810 along the channel between the channel locationcorresponding with dummy word line WLD and the bottom of the channel maybe set based on the unselected word line voltage applied to the dummyword line WLD and the channel resistance between the channel locationcorresponding with dummy word line WLD and the bottom of the channel. Insome embodiments, the slope 809 corresponding with an average channelvoltage along the channel between the channel location correspondingwith word line WL15 and the channel location corresponding with dummyword line WLD may be adjusted based on the selected select gate voltageapplied to the select gate SG 836 (e.g., the slope may be increased byapplying a higher select gate voltage to the select gate SG 836; anincreased slope corresponds with a smaller voltage drop across thechannel of an adjustable resistance local bit line).

FIG. 8P depicts one embodiment of word line voltage settings that may beused during the RESET operation depicted in FIG. 8O. As depicted, theselected word line WL15 has been set to 0V, the dummy word line WLD hasbeen set to 2.6V in order to set the channel voltage at a channellocation corresponding with word line WL0 to be close to orsubstantially the same as 2.5V, the unselected word lines WL0-WL6 havebeen set to 2.0V, and the unselected word lines WL7-WL14 have been setto 1.5V. In one embodiment, a first set of unselected word lines (e.g.,word lines WL0-WL6) may be set to a first unselected word line voltageand a second set of unselected word lines (e.g., word lines WL7-WL14)may be set to a second unselected word line voltage different from thefirst unselected word line voltage. The first set of unselected wordlines may comprise word lines that are closer to the dummy word linethan the second set of unselected word lines. The first unselected wordline voltage may be greater than the second unselected word linevoltage. In the case depicted in FIG. 8P, a voltage difference acrossthe memory cell connected to word line WL0 may comprise 0.5V (i.e.,2.5V-2.0V) and the voltage difference across the memory cell connectedto word line WL7 may comprise 0.5V (i.e., 2.0V-1.5V). Although theunselected word lines WL0-WL14 have been partitioned into two sets inFIG. 8P, in other cases, the unselected word lines WL0-WL14 may bepartitioned into three or more sets of unselected word lines (e.g.,WL0-WL3 may form a first set, WL4-WL7 may form a second set, WL8-WL11may form a third set, and WL12-WL14 may form a fourth set). Each of thethree or more sets of unselected word lines may be biased to a differentunselected word line voltage.

FIG. 8Q depicts another embodiment of word line voltage settings thatmay be used during the RESET operation depicted in FIG. 8O. As depicted,the selected word line WL15 has been set to 0V, the dummy word line WLDhas been set to 2.6V in order to set the channel voltage at a channellocation corresponding with word line WL0 to be close to orsubstantially the same as 2.5V, the unselected word line WL0 has beenset to 2.5V, and the other unselected word lines WL1-WL14 have been setto voltages based on the average channel voltage along the channelbetween the channel location corresponding with word line WL15 and thechannel location corresponding with word line WL1. In one embodiment,the unselected word lines WL0-WL14 may be assigned voltages based on theslope 809 representing an average channel voltage along the channelbetween the channel location corresponding with word line WL15 and thechannel location corresponding with word line WLD. In some cases, thevoltages applied to the unselected word lines WL0-WL14 may be set inorder to minimize the voltage stress across each of the memory cellsconnected to the unselected word lines WL0-WL14. In one example, thevoltages applied to unselected word lines may be set in order to place0V or substantially close to 0V across each of the memory cellsconnected to the unselected word lines. Setting the unselected wordlines to location-dependent voltages that minimize the voltagedifferences across unselected memory cells may reduce program disturband improve memory cell reliability. As depicted, the unselected wordline voltages applied to the unselected word lines between word lineWL15 and word line WL0 depend on the location of the unselected wordline. For example, the unselected word line voltage applied to word lineWL1 may comprise 2.4V and the unselected word line voltage applied toword line WL14 may comprise 1.6V.

In some embodiments, the ability to minimize the voltage differencesacross unselected memory cells (e.g., F cells) connected to anadjustable resistance local bit line may allow higher write currents tobe used during a programming operation. Using higher write currents mayallow selected memory cells to be programmed in a shorter amount of time(i.e., enable faster write times). During a RESET operation, theunselected word lines may be biased at different voltages such thatvoltage drop across each of the unselected memory cells is minimizedbefore the selected memory cell is actually RESET into a high resistancestate.

FIG. 8R depicts one embodiment of an adjustable resistance bit linestructure during a SET operation after a selected memory cell has beenSET into a low resistance state (i.e., post-SET). As depicted, theselected word line WL15 has been set to a selected word line voltage(e.g., 5V) and the selected global bit line GBL 804 has been set to aselected bit line voltage (e.g., 0V). The select gate SG 836 may be setto a selected select gate voltage such that the adjustable resistancelocal bit line AR_LBL 816 is set into a conducting state. However,although the adjustable resistance local bit line AR_LBL 816 is set intothe conducting state, a channel resistance of a channel of theadjustable resistance local bit line AR_LBL 816 may cause voltage dropsto occur along the channel. As depicted, the channel voltage at thebottom of the channel is 0V, the channel voltage at a channel locationcorresponding with word line WL0 is 0.5V, and the channel voltage at achannel location corresponding with word line WL15 is 2.5V. The voltageacross the selected memory cell may comprise the difference between theselected word line voltage and the channel voltage at the channellocation corresponding with word line WL15 (i.e., 2.5V). The slope 806of the channel voltage between the bottom of the channel and the channellocation corresponding with the selected word line WL15 may be adjustedbased on the selected select gate voltage applied to the select gate SG836. The slope 806 may also be adjusted based on the current through theselected memory cell (e.g., 100 nA, 2 μA, or 50 μA).

FIG. 8S depicts one embodiment of word line voltage settings that may beused during the SET operation depicted in FIG. 8R. As depicted, theselected word line WL15 has been set to 5V, the unselected word linesWL0-WL6 have been set to 1.0V, the unselected word lines WL7-WL14 havebeen set to voltages corresponding with the slope 806 of the channelvoltage between the bottom of the channel and the channel locationcorresponding with the selected word line WL15. In this case, thevoltage difference across the memory cell connected to word line WL1 maycomprise 0.5V (i.e., 1.0V-0.5V) and the voltage difference across thememory cell connected to word line WL14 may comprise 0V (i.e.,2.25V-2.25V). In some embodiments, the voltages applied to theunselected word lines WL0-WL14 may be set in order to minimize thevoltage stress across each of the memory cells connected to theunselected word lines WL0-WL14. In one example, the voltages applied tounselected word lines may be set in order to place 0V or substantiallyclose to 0V across each of the memory cells connected to the unselectedword lines. Setting the unselected word lines to location-dependentvoltages that minimize the voltage differences across unselected memorycells may reduce program disturb and improve memory cell reliability.

In some embodiments, the voltages applied to unselected word linesduring a SET operation may be set in order to place at most a firstvoltage (e.g., at most 500 mV or at most 1V) across each of the memorycells connected to the unselected word lines. In other embodiments, thevoltages applied to a first set of unselected word lines during a SEToperation may be set in order to place at most a first voltage (e.g., atmost 500 mV or at most 1V) across each of the memory cells connected tothe first set of unselected word lines. In one example, if a selectedword line comprises word line WL5, then the first set of word lines maycomprise word lines WL0-WL4.

FIG. 8T depicts another embodiment of word line voltage settings thatmay be used during the SET operation depicted in FIG. 8R. As depicted,the selected word line WL15 has been set to 5V, the unselected wordlines WL0-WL6 have been set to 0.5V, and the unselected word linesWL7-WL14 have been set to 1.5V. In one embodiment, a first set ofunselected word lines may be set to a first voltage and a second set ofthe unselected word lines may be set to a second voltage greater thanthe first voltage. The first set of unselected word lines may compriseword lines that are closest to the bottom of the adjustable resistancebit line structure or closest to the global bit line located at thebottom of the adjustable resistance bit line structure.

In some embodiments, during a SET operation, the unselected word linesmay be biased at different voltages such that the voltage drop acrosseach of the unselected memory cells is minimized after the selectedmemory cell is actually SET into a low resistance state. In one example,the unselected word lines WL0-WL14 may be assigned voltages based on theslope 806 of the channel voltage between the bottom of the channel andthe channel location corresponding with the selected word line WL15.

FIG. 8U depicts one embodiment of an adjustable resistance bit linestructure during a read operation. As depicted, a dummy word line WLDpositioned as the bottommost word line closest to the global bit lineconnection has been set to 1.5V, the selected word line WL15 has beenset to a selected word line voltage for a read operation (e.g., 0V) andthe selected global bit line GBL 804 has been set to a selected bit linevoltage for a read operation (e.g., 3V). The dummy word line WLD may beset to 1.5V in order to set the channel voltage at the channel locationcorresponding with word line WL0 to be close to or substantially thesame as 1.4V. The select gate SG 836 may be set to a selected selectgate voltage such that the adjustable resistance local bit line AR_LBL816 is set into a conducting state. However, although the adjustableresistance local bit line AR_LBL 816 is set into the conducting state, achannel resistance of a channel of the adjustable resistance local bitline AR_LBL 816 may cause voltage drops to occur along the channelduring the read operation. As depicted, the channel voltage at thebottom of the channel is 3V, the channel voltage at a channel locationcorresponding with dummy word line WLD is 1.5V, the channel voltage at achannel location corresponding with word line WL0 is 1.4V, and thechannel voltage at a channel location corresponding with word line WL15is 1.0V. The voltage across the selected memory cell may comprise thedifference between the selected word line voltage and the channelvoltage at the channel location corresponding with word line WL15 (i.e.,1.0V).

The slope of the channel voltage between the bottom of the channel andthe channel location corresponding with bottommost word line may be setbased on an unselected word line voltage applied to the word line WLD.In one example, if the unselected word line voltage applied to the dummyword line WLD is 1.5V, then the channel voltage at the channel locationcorresponding with word line WLD may be set to 1.5V or to a voltagesubstantially close to 1.5V. The slope 811 of the channel voltagebetween the channel location corresponding with word line WL15 and thechannel location corresponding with word line WLD may be set based onthe unselected word line voltage applied to the word line WLD and theconductivity of the channel. The slope 811 may depend on the selectedselect gate voltage applied to the select gate SG 836. The slope 811 mayalso depend on the current through the selected memory cell being sensedduring the read operation (e.g., 20 nA or 100 nA). In some embodiments,the slope 811 corresponding with the channel voltage along the channelbetween the channel location corresponding with word line WL15 and thechannel location corresponding with dummy word line WLD may be adjustedbased on the selected select gate voltage applied to the select gate SG836 (e.g., the slope may be increased by applying a higher select gatevoltage to the select gate SG 836).

FIG. 8V depicts one embodiment of word line voltage settings that may beused during the read operation depicted in FIG. 8U. As depicted, theselected word line WL15 has been set to 0V, the dummy word line WLD hasbeen set to 1.5V in order to set the channel voltage at a channellocation corresponding with word line WL0 to be close to orsubstantially the same as 1.4V, and the unselected word lines WL0-WL14have been set to 1.25V. In one embodiment, a first set of unselectedword lines (e.g., word lines WL0-WL6) may be set to a first unselectedword line voltage and a second set of unselected word lines (e.g., wordlines WL7-WL14) may be set to a second unselected word line voltagedifferent from the first unselected word line voltage. The first set ofunselected word lines may comprise word lines that are closer to thedummy word line than the second set of unselected word lines. The firstunselected word line voltage may be greater than the second unselectedword line voltage. In the case depicted in FIG. 8V, a voltage differenceacross the memory cell connected to word line WL0 may comprise 0.15V(i.e., 1.4V-1.25V) and the voltage difference across the memory cellconnected to word line WL7 may comprise 0V (i.e., 1.25V-1.25V).

FIG. 8W depicts another embodiment of word line voltage settings thatmay be used during the read operation depicted in FIG. 8U. As depicted,the selected word line WL15 has been set to 0V, the dummy word line WLDhas been set to 1.5V, and the unselected word lines WL0-WL14 have beenset to voltages based on the average channel voltage along the channelbetween the channel location corresponding with word line WL15 and thechannel location corresponding with dummy word line WLD. In one example,the unselected word lines WL0-WL14 may be assigned voltages based on theslope 811 representing an average channel voltage along the channelbetween the channel location corresponding with word line WL15 and thechannel location corresponding with dummy word line WLD. In some cases,the voltages applied to the unselected word lines WL0-WL14 may set inorder to minimize the voltage stress across each of the memory cellsconnected to the unselected word lines WL0-WL14 during a read operation.In one example, the voltages applied to the unselected word lines may beset in order to place 0V or substantially close to 0V across each of thememory cells connected to the unselected word lines. Setting theunselected word lines to location-dependent voltages that minimize thevoltage differences across unselected memory cells may reduce readdisturb and improve memory cell reliability. As depicted, the unselectedword line voltages applied to the unselected word lines between wordline WL15 and word line WL0 depend on the location of the unselectedword line.

In some embodiments, the voltages applied to unselected word linesduring a read operation, such as the unselected word lines WL0-WL14 inFIG. 8W, may be set in order to place at most a first voltage (e.g., atmost 100 mV or at most 200 mV) across each of the memory cells connectedto the unselected word lines. In other embodiments, the voltages appliedto a first set of unselected word lines during a read operation may beset in order to place at most a first voltage (e.g., at most 50 mV or atmost 100 mV) across each of the memory cells connected to the first setof unselected word lines. In one example, if a selected word linecomprises word line WL8, then the first set of word lines may compriseword lines WL0-WL7.

In some embodiments, the ability to minimize the voltage differencesacross unselected memory cells (e.g., F cells) connected to anadjustable resistance local bit line may allow higher read currents tobe used during a read operation. Using higher read currents may allowselected memory cells to be sensed or read in a shorter amount of time(i.e., enable faster read times).

FIG. 9A depicts one embodiment of a circuit for generating unselectedword line voltages. The circuit may comprise an auto-tracking unselectedword line voltage generator that generates different word line voltagesbased on a word line location or based on the position of a word linealong an adjustable resistance local bit line. As depicted, the circuitfor generating unselected word line voltages includes a replica bit linestructure 903. The replica bit line structure 903 includes a select gateSG 902 for controlling an adjustable resistance local bit line. A globalbit line GBL 904 connects to the adjustable resistance local bit line.The adjustable resistance local bit line connects to a plurality ofmemory cells, such as memory cell 901. The plurality of memory cellsconnect to VUX reference lines VUX_REF0-VUX_REF4. Each VUX referenceline connects to a bidirectional analog mux that is connected to anon-inverting amplifier with a configurable voltage divider that isconnected to a unity gain buffer stage for driving one of the unselectedword line voltages generated.

In one embodiment, a replica bit line structure, such as replica bitline structure 903, may replicate an adjustable resistance bit linestructure in which a global bit line connection is made at the bottom ornear the bottom of the adjustable resistance bit line structure, such asthe adjustable resistance bit line structure depicted in FIG. 6D.Although the global bit line GBL 904 connects to the adjustableresistance local bit line at the bottom of the replica bit linestructure 903 depicted in FIG. 9A, in other embodiments, a global bitline may connect to a replicated adjustable resistance local bit line atthe top or near the top of a replica bit line structure. As an example,a replica bit line structure may replicate an adjustable resistance bitline structure in which a global bit line connection is made at the topor near the top of the adjustable resistance bit line structure, such asthe adjustable resistance bit line structure depicted in FIG. 6V.

As depicted in FIG. 9A, VUX reference line VUX_REF4 connects tobidirectional analog mux 910. The bidirectional analog mux 910 mayinclude a pair of transmission gates (or T-gates) for selectivelyconnecting the output of the bidirectional analog mux to one of theinputs of the bidirectional analog mux. The bidirectional analog mux 910may selectively connect either a selected word line voltage SWL 905 tothe VUX reference line VUX_REF4 or connect the VUX reference lineVUX_REF4 to an input of amplifier 909 that is configured in anon-inverting amplifier configuration with a resistive voltage dividerformed by resisters 906-907. In some cases, the non-inverting amplifierconfiguration may allow a voltage of the VUX reference line VUX_REF4 tobe level shifted prior to being buffered by a unity gain buffer. Theoutput of the amplifier 909 drives an input of amplifier 908 that isconfigured in a unity gain amplifier configuration. The output of theunity gain buffer is an unselected word line voltage VUX_WL4 for drivinga word line WL4.

Also depicted, VUX reference line VUX_REF0 connects to bidirectionalanalog mux 920. The bidirectional analog mux 920 may selectively connecteither a selected word line voltage SWL 905 to the VUX reference lineVUX_REF0 or connect the VUX reference line VUX_REF0 to an input ofamplifier 919 that is configured in a non-inverting amplifierconfiguration with a resistive voltage divider formed by resisters916-917. The output of the amplifier 919 drives an input of amplifier918 that is configured in a unity gain amplifier configuration. Theoutput of the unity gain buffer is an unselected word line voltageVUX_WL0 for driving a word line WL0.

In one embodiment, the memory cells of the replica bit line structure903 may be set into a high resistance state prior to generating theunselected word line voltages. In another embodiment, the memory cellsof the replica bit line structure 903 may be set into a low resistancestate less than the high resistance state prior to generating theunselected word voltages. In another embodiment, the memory cells of thereplica bit line structure 903 may all be RESET prior to generating theunselected word voltages. In another embodiment, the memory cells of thereplica bit line structure 903 may all be SET prior to generating theunselected word voltages.

In one embodiment, if a word line corresponding with word line WL4comprises a selected word line, then the bidirectional analog mux 910may be configured to connect the selected word line voltage SWL 905 tothe VUX reference line VUX_REF4 and the bidirectional analog mux 920 maybe configured to connect the VUX reference line VUX_REF0 to the input ofamplifier 919. The bidirectional analog multiplexors connected to VUXreference lines VUX_REF1-VUX_REF3 may also be configured to connect theVUX reference lines VUX_REF1-VUX_REF3 to non-inverting amplifierconfigurations. In another embodiment, if a word line corresponding withword line WL3 comprises a selected word line, then a bidirectionalanalog mux not depicted may be configured to connect the selected wordline voltage SWL 905 to the VUX reference line VUX_REF3 and thebidirectional analog mux 920 may be configured to connect the VUXreference line VUX_REF0 to the input of amplifier 919. In some cases, abidirectional analog mux associated with a selected word line may drivethe corresponding VUX reference line to the selected word line voltageSWL 905 and the other bidirectional analog multiplexors associated withunselected word lines may connect the other VUX reference lines tonon-inverting amplifier configurations for generating unselected wordline voltages.

In some embodiments, in order to generate unselected word line voltagesfor word lines WL0-WL3 during a RESET operation when a selected memorycell connected to word line WL4 is to be selected, the global bit lineGBL 904 may be set to a selected bit line voltage (e.g., 5V), the VUXreference line VUX_REF4 may be set to the selected word line voltage(e.g., 0V), and the select gate SG 902 may be set to the selected selectline voltage (e.g., 7V) in order to set the adjustable resistance localbit line of the replica bit line structure 903 into a conducting state.In this case, the channel voltages generated along the channel betweenthe global bit line GBL 904 and the selected memory cell connected tothe VUX reference line VUX_REF4 may be outputted and buffered usingunity gain buffers. The non-inverting amplifier configurations may beused to adjust or level shift the VUX reference voltages (e.g., todecrease a VUX reference voltage based on a configuration of a resistivevoltage divider) prior to being buffered by the unity gain buffers.

In some embodiments, in order to generate unselected word line voltagesfor word lines WL0-WL3 during a SET operation when a selected memorycell connected to word line WL4 is to be selected, the global bit lineGBL 904 may be set to a selected bit line voltage (e.g., 0V), the VUXreference line VUX_REF4 may be set to the selected word line voltage(e.g., 5V), and the select gate SG 902 may be set to the selected selectline voltage (e.g., 5V) in order to set the adjustable resistance localbit line of the replica bit line structure 903 into a conducting state.In this case, the channel voltages generated along the channel betweenthe global bit line GBL 904 and the selected memory cell connected tothe VUX reference line VUX_REF4 may be outputted and buffered usingunity gain buffers. The non-inverting amplifier configurations may beused to adjust or level shift the VUX reference voltages associated withword lines WL0-WL3 (e.g., to decrease a VUX reference voltage associatedwith VUX reference line VUX_REF1 based on a configuration of a resistivevoltage divider) prior to being buffered by the unity gain buffers.

In some embodiments, in order to generate unselected word line voltagesduring a read operation when a selected memory cell connected to wordline WL4 is to be selected, the global bit line GBL 904 may be set to aselected bit line voltage (e.g., 3V), the VUX reference line VUX_REF4may be set to the selected word line voltage (e.g., 0V), and the selectgate SG 902 may be set to the selected select line voltage for readoperations (e.g., 4V) in order to set the adjustable resistance localbit line of the replica bit line structure 903 into a conducting state.In this case, the channel voltages generated along the channel betweenthe global bit line GBL 904 and the selected memory cell connected tothe VUX reference line VUX_REF4 may be outputted and buffered usingunity gain buffers. The non-inverting amplifier configurations may beused to adjust or level shift the VUX reference voltages associated withword lines WL0-WL3 prior to being buffered by the unity gain buffers(e.g., level shifting the VUX reference voltages by −150 mV).

FIGS. 9B-9C depict a flowchart describing one embodiment of a processfor performing a programming operation. In one embodiment, the processof FIGS. 9B-9C may be performed by a memory system, such as memorysystem 101 in FIG. 1A.

In step 942, a plurality of data to be programmed is acquired. Theplurality of data to be programmed may be associated with one or morepages of data or a portion of a page of data. In step 944, a first wordline within a memory array is determined. The first word line maycorrespond with a word line to be selected during a programmingoperation. In step 946, a first global bit line within the memory arrayis determined. The first global bit line may be connected to anadjustable resistance bit line structure that includes an adjustableresistance local bit line and a select gate. A first memory cell may bearranged between the adjustable resistance local bit line and the firstword line.

In step 948, a maximum current limit for the first memory cell isdetermined. The maximum current limit may be determined based on whetherthe programming operation comprises a SET or RESET operation. In step950, a selected select gate voltage is determined based on the maximumcurrent limit. In step 952, the adjustable resistance local bit line isset into a conducting state by applying the selected select gate voltageto the select gate. In step 954, a plurality of other word lines withinthe memory array is determined. A plurality of unselected memory cellsmay be arranged between the adjustable resistance local bit line and theplurality of other word lines. In step 956, one or more unselected wordline voltages are generated based on a position of the first word linerelative to the plurality of other word lines. In one embodiment, theone or more unselected word line voltages may be generated using thecircuit depicted in FIG. 9A. In step 958, each word line of theplurality of other word lines is set to one of the one or moreunselected word line voltages.

In step 960, a dummy word line within the memory array is determined.The dummy word line may comprise the word line closest to the firstglobal bit line or the word line closest to the bottom of the adjustableresistance bit line structure. A dummy memory cell connected to thedummy word line and the adjustable resistance local bit line may be leftin an unformed state or set into a high resistance state. In step 962,the dummy word line is set to a dummy word line voltage. In oneembodiment, the dummy word line may be set to the dummy word linevoltage in order to adjust a channel voltage corresponding with aposition of the dummy word line. In some embodiments, the dummy wordline may be set to the dummy word line voltage prior to applying theselected select gate voltage to the select gate. In some cases, thedummy word line may be set to the dummy word line voltage prior to theselect gate being set to the selected select gate voltage and/or priorto the plurality of other word lines being set to the one or moreunselected word line voltages in order to prevent or reduce surgecurrent through memory cells.

In step 964, a programming operation is performed on the memory array toprogram at least a first portion of the plurality of data into thememory array. The programming operation may include applying a selectedword line voltage to the first word line and applying a selected bitline voltage to the first global bit line while the adjustableresistance local bit line is set into the conducting state.

In some embodiments, if a programming operation comprises a SEToperation, then unselected word lines may be biased at differentvoltages prior to and/or during a selected memory cell being SET into alow resistance state in order to minimize the voltage stress placedacross each of the unselected memory cells associated with theunselected word lines that occurs subsequent to the selected memory cellactually being SET into the low resistance state. In one example, if theprogramming operation comprises a SET operation, then unselected wordline voltages may be generated and applied to the unselected word linesprior to the selected memory cell being SET into the low resistancestate in order to minimize the voltage stress placed across each of theunselected memory cells after the selected memory cell has been SET intothe low resistance state.

In other embodiments, if a programming operation comprises a RESEToperation, then unselected word lines may be biased at differentvoltages prior to and/or during a selected memory cell being RESET intoa high resistance state in order to minimize the voltage stress placedacross each of the unselected memory cells associated with theunselected word lines that occurs prior to the selected memory cellactually being RESET into the high resistance state. In one example, ifthe programming operation comprises a RESET operation, then unselectedword line voltages may be generated and applied to the unselected wordlines prior to the selected memory cell being RESET into the highresistance state in order to minimize the voltage stress placed acrosseach of the unselected memory cells before the selected memory cell hasbeen RESET into the high resistance state.

In some embodiments, the voltages applied to a set of unselected wordlines during a programming operation may be set in order to place atmost a first voltage (e.g., at most 500 mV or at most 1.2V) across eachof the memory cells connected to the set of unselected word lines.

FIG. 9D depicts a flowchart describing one embodiment of a process forperforming a read operation. In one embodiment, the process of FIG. 9Dmay be performed by a memory system, such as memory system 101 in FIG.1A.

In step 972, a first word line within a memory array is determined. Thefirst word line may correspond with a word line to be selected during aread operation. In step 974, a first global bit line within the memoryarray is determined. The first global bit line may be connected to anadjustable resistance bit line structure that includes an adjustableresistance local bit line and a select gate. A first memory cell may bearranged between the adjustable resistance local bit line and the firstword line. In step 976, the adjustable resistance local bit line is setinto a conducting state by applying a selected select gate voltage tothe select gate. In step 978, a plurality of other word lines within thememory array is determined. A plurality of unselected memory cells maybe arranged between the adjustable resistance local bit line and theplurality of other word lines. In step 980, one or more unselected wordline voltages are generated based on a position of the first word linerelative to the plurality of other word lines. In one embodiment, theone or more unselected word line voltages may be generated using thecircuit depicted in FIG. 9A. In step 982, each word line of theplurality of other word lines is set to one of the one or moreunselected word line voltages. In step 984, a read operation isperformed on the memory array. The read operation may include applying aselected word line voltage to the first word line and applying aselected bit line voltage to the first global bit line while theadjustable resistance local bit line is set into the conducting state.

In some embodiments, unselected word lines may be biased at differentvoltages during a read operation in order to minimize the voltage stressplaced across each of the unselected memory cells associated with theunselected word lines that occurs during the read operation. In somecases, the voltages applied to a set of unselected word lines during aread operation may be set in order to place at most a first voltage(e.g., at most 50 mV or at most 100 mV) across each of the memory cellsconnected to the set of unselected word lines.

FIGS. 10A-10P depict various embodiments of cross-sectional viewsrelated to processes for fabricating a portion of an adjustableresistance bit line structure. As depicted in FIGS. 10A-10B, analternating stack of word line layers and dielectric layers, such asword line layer 242 and dielectric layer 243, have been formed over asubstrate. FIG. 10B depicts one embodiment of a cross-sectional viewtaken along line X-X of FIG. 10A. The word line layer 242 may compriseTiN, polysilicon, or tungsten (W). The dielectric layer 243 may comprisean oxide layer. As depicted in FIGS. 10C-10D, a trench that was etchedextending through a plurality of word line layers and dielectric layershas been filled with oxide in order to form an oxide partition 244 thatextends through the plurality of word line layers and dielectric layers.FIG. 10D depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 10C.

As depicted in FIGS. 10E-10F, a memory hole 246 has been etchedextending through a plurality of word line layers and a plurality ofdielectric layers. FIG. 10F depicts one embodiment of a cross-sectionalview taken along line X-X of FIG. 10E. In some embodiments, a pluralityof memory holes may be formed by etching through an alternating stack ofword line layers and dielectric layers (e.g., etching through layers ofTiN or polysilicon that are separated by oxide layers) to form theplurality of memory holes. The plurality of memory holes may compriserectangular, square, or cylindrical holes. The plurality of memory holesmay be formed by patterning and then removing material using variousetching techniques such as dry etching, wet chemical etching, plasmaetching, or reactive-ion etching (RIE). In some cases, the selectiveremoval of material may be performed using a lithography sequenceincluding depositing a layer of photoresist (positive or negative) overthe material, exposing the layer of photoresist to light via a mask(i.e., the mask determines which areas of the layer of photoresist areexposed to the light), and then selectively etching the material basedon the exposed portions of the layer of photoresist.

As depicted in FIGS. 10G-10H, a memory element layer 247 (e.g.,comprising a ReRAM material) and an intrinsic polysilicon region 248have been deposited within the memory hole 246. In one embodiment, thememory element layer 247 may comprise a conformal layer of memoryelement material surrounding the sides of the memory hole 246 and theintrinsic polysilicon region 248 may comprise a conformal layer ofintrinsic polysilicon that has been deposited on a surface of theconformal layer of memory element material. The conformal layer ofintrinsic polysilicon may have a thickness (or width) of 10 nm to 20 nm.As depicted, a narrow hole 241 has been formed such that the intrinsicpolysilicon region 248 surrounds the narrow hole 241 in two dimensions.FIG. 10H depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 10G. In one example, the memory element layer 247 maybe formed within the memory hole 246 and the intrinsic polysiliconregion 248 may be formed over the memory element layer 247. The memoryelement layer 247 and/or the intrinsic polysilicon region 248 may bedeposited within the memory hole 246 using various deposition techniquessuch as chemical vapor deposition (CVD), physical vapor deposition(PVD), or atomic layer deposition (ALD). The memory element layer 247may comprise a phase change material, a ferroelectric material, or ametal oxide such as nickel oxide or hafnium oxide. The intrinsicpolysilicon region 248 may comprise undoped polysilicon, undoped silicongermanium, or undoped indium gallium arsenide. In some cases, the memoryelement layer 247 may have a thickness (or width) of 2 nm to 4 nm andthe intrinsic polysilicon region 248 may have a thickness (or width) of7 nm to 20 nm.

As depicted in FIGS. 10I-10J, a dielectric layer 249 and a select gateregion 250 have been formed within the narrow hole 241. FIG. 10J depictsone embodiment of a cross-sectional view taken along line X-X of FIG.10I. In one embodiment, the dielectric layer 249 may comprise aconformal layer of silicon dioxide coating an inside surface of theintrinsic polysilicon region 248. In some embodiments, the dielectriclayer 249 and the select gate region 250 may be formed within the narrowhole 241 by depositing the dielectric layer 249 and the select gateregion 250 using various deposition techniques such as chemical vapordeposition (CVD), physical vapor deposition (PVD), or atomic layerdeposition (ALD). The select gate region 250 may comprise TiN orpolysilicon. The dielectric layer 249 may comprise an oxide, silicondioxide, silicon nitride, or a high-k dielectric material. In somecases, the dielectric layer 249 may have a thickness (or width) of 5 nmto 10 nm.

As depicted in FIGS. 10K-10L, an oxide layer 260 and a global bit line261 have been formed above the select gate region 250. FIG. 10L depictsone embodiment of a cross-sectional view taken along line X-X of FIG.10K. The global bit line 261 may comprise TiN or tungsten. As depictedin FIGS. 10M-10N, a top pillar hole 262 has been etched above the selectgate region 250. FIG. 10N depicts one embodiment of a cross-sectionalview taken along line X-X of FIG. 10M. As depicted in FIGS. 10O-10P, N+polysilicon region 266, N+ polysilicon region 265, and oxide region 264may be formed within the top pillar hole 262. FIG. 10P depicts oneembodiment of a cross-sectional view taken along line X-X of FIG. 10O.The N+ polysilicon region 266 may provide an electrical connection fromthe intrinsic polysilicon region 248 to the global bit line 261. In oneembodiment, a layer of N+ polysilicon may be deposited within the toppillar hole 262, etched, and then filled with oxide in order to form theoxide region 264, the N+ polysilicon region 266, and the N+ polysiliconregion 265. In another embodiment, the N+ polysilicon region 266 maycomprise a conformal layer of N+ polysilicon surrounding the sides ofthe top pillar hole 262 and the oxide region 264 may comprise aconformal layer of silicon dioxide that has been deposited on a surfaceof the conformal layer of N+ polysilicon.

In some embodiments, a distributed NMOS FET structure may be formed byusing an N+ polysilicon region, such as N+ polysilicon region 266,directly connected to the intrinsic polysilicon region 248. In otherembodiments, a distributed PMOS FET structure may be formed by using aP+ polysilicon region in place of the N+ polysilicon region 266.

FIGS. 10Q-10X depict various embodiments of cross-sectional viewsrelated to processes for fabricating a portion of an adjustableresistance bit line structure. As depicted in FIGS. 10Q-10R, adielectric layer 249 and a select gate region 250 have been formedwithin the intrinsic polysilicon region 248. An oxide layer 270 has beenovergrown or formed to have a thickness that is greater than thedielectric layer 243. FIG. 10R depicts one embodiment of across-sectional view taken along line X-X of FIG. 10Q.

As depicted in FIGS. 10S-10T, a portion of the oxide layer 270 and aportion of the memory element layer 247 have been etched to expose a topportion of the intrinsic polysilicon region 248. FIG. 10T depicts oneembodiment of a cross-sectional view taken along line X-X of FIG. 10S.As depicted in FIGS. 10U-10V, the top portion of the intrinsicpolysilicon region 248 has been doped in order to form N+ polysiliconregion 271. FIG. 10V depicts one embodiment of a cross-sectional viewtaken along line X-X of FIG. 10U. In some cases, the N+ polysiliconregion 271 may be formed via diffusion or ion implantation. The topportion of the intrinsic polysilicon region 248 may be doped with ann-type dopant (e.g., phosphorus or arsenic).

In some embodiments, a distributed NMOS FET structure may be formed byusing an N+ polysilicon region, such as N+ polysilicon region 271,directly connected to the intrinsic polysilicon region 248. In otherembodiments, a distributed PMOS FET structure may be formed by using aP+ polysilicon region in place of the N+ polysilicon region 271. In somecases, a P+ polysilicon region may be formed via diffusion or ionimplantation. The top portion of the intrinsic polysilicon region 248may be doped with a p-type dopant (e.g., boron).

As depicted in FIGS. 10W-10X, an oxide layer 272 and a global bit line273 have been formed. FIG. 10X depicts one embodiment of across-sectional view taken along line X-X of FIG. 10W. The global bitline 273 may comprise TiN or tungsten. The oxide layer 272 may comprisean oxide or silicon dioxide. The N+ polysilicon region 271 may providean electrical connection from the intrinsic polysilicon region 248 tothe global bit line 273.

Some embodiments of the disclosed technology include systems and methodsfor reducing the number of unselected memory cells (e.g., H-cells orU-cells) or the amount of leakage current through unselected memorycells during read and/or write operations using an intrinsic verticalbit line architecture (iVBL). The iVBL architecture may improve memoryperformance and provide low current operation for a non-volatile memoryarray, such as a ReRAM array. In some cases, the iVBL architecture mayeliminate or significantly reduce the leakage currents through H-cellsby making unselected vertical bit lines connected to the H-cells highlyresistive (e.g., more than 1 Gohm) or non-conducting and making selectedvertical bit lines low resistance (e.g., less than 1 Kohm) or conducting(e.g., only a selected vertical bit line connected to a selected memorycell may be made conductive, while all other vertical bit lines may bemade non-conductive).

One embodiment of the disclosed technology includes a first word line,an adjustable resistance bit line structure, and a first memory elementarranged between the first word line and the adjustable resistance localbit line. The adjustable resistance bit line structure includes anadjustable resistance local bit line and a select gate. The adjustableresistance bit line structure configured to set a resistance of theadjustable resistance local bit line based on a first voltage applied tothe select gate. In some cases, the adjustable resistance bit linestructure may be configured to set the adjustable resistance local bitline into a non-conducting state based on the first voltage applied tothe select gate. The adjustable resistance local bit line may compriseintrinsic polysilicon.

One embodiment of the disclosed technology includes a first word line, aglobal bit line, a vertical bit line structure including a layer ofintrinsic polysilicon and a select gate, and a first memory elementarranged between the first word line and the layer of intrinsicpolysilicon. The layer of intrinsic polysilicon connected to the globalbit line. The vertical bit line structure configured to set a resistanceof the layer of intrinsic polysilicon based on a first voltage appliedto the select gate.

One embodiment of the disclosed technology includes a first word line, aglobal bit line, a first adjustable resistance bit line structureincluding a first adjustable resistance local bit line connected to theglobal bit line, a first memory element arranged between the first wordline and the first adjustable resistance local bit line, a second wordline, a second adjustable resistance bit line structure including asecond adjustable resistance local bit line connected to the global bitline, and a second memory element arranged between the second word lineand the second adjustable resistance local bit line. The firstadjustable resistance bit line structure configured to set the firstadjustable resistance local bit line into a conducting state during amemory operation. The second adjustable resistance bit line structureconfigured to set the second adjustable resistance local bit line into anon-conducting state during the memory operation.

One embodiment of the disclosed technology includes identifying a firstword line within a memory array and identifying a first global bit linewithin the memory array. The first global bit line is connected to anadjustable resistance bit line structure. The adjustable resistance bitline structure includes an adjustable resistance local bit line and aselect gate. A first memory cell is arranged between the adjustableresistance local bit line and the first word line. The method furthercomprises setting the adjustable resistance local bit line into aconducting state by applying a first voltage to the select gate andperforming a memory operation on the memory array. The memory operationincludes applying a selected word line voltage to the first word lineand a selected bit line voltage to the first global bit line while theadjustable resistance local bit line is set into the conducting state.In some cases, the memory operation may comprise a programmingoperation, a SET operation, a RESET operation, or a read operation.

One embodiment of the disclosed technology includes a memory array andone or more managing circuits. The memory array includes a firstadjustable resistance bit line structure and a second adjustableresistance bit line structure. The first adjustable resistance bit linestructure connected to a first set of memory cells. The secondadjustable resistance bit line structure connected a second set ofmemory cells. The one or more managing circuits in communication withthe first adjustable resistance bit line structure and the secondadjustable resistance bit line structure. The one or more managingcircuits configured to set the first adjustable resistance bit linestructure into a conducting state and the second adjustable resistancebit line structure into a non-conducting state during a memoryoperation.

One embodiment of the disclosed technology includes determining a firstword line within a memory array and determining a first global bit linewithin the memory array. The first global bit line is connected to anadjustable resistance bit line structure that includes an adjustableresistance local bit line and a select gate. A first memory cell isarranged between the adjustable resistance local bit line and the firstword line. The method further comprises determining a dummy word linewithin the memory array that comprises the word line closest to thefirst global bit line, determining a dummy word line voltage, andperforming a memory operation on the memory array. The memory operationincludes applying a selected word line voltage to the first word lineand applying a selected bit line voltage to the first global bit linewhile the adjustable resistance local bit line is set into a conductingstate. The memory operation includes applying the dummy word linevoltage to the dummy word line while the adjustable resistance local bitline is set into the conducting state.

One embodiment of the disclosed technology includes identifying a dummyword line within a memory array. The memory array includes a first wordline and a first global bit line. The first global bit line is connectedto an adjustable resistance bit line structure that includes anadjustable resistance local bit line and a select gate. A first memorycell is arranged between the adjustable resistance local bit line andthe first word line. The dummy word line comprises the word line closestto the first global bit line. The method further comprises determining adummy word line voltage, determining a maximum current limit for thefirst memory cell, determining a selected select gate voltage based onthe maximum current limit, and performing a memory operation on thememory array. The memory operation includes applying a selected wordline voltage to the first word line and applying a selected bit linevoltage to the first global bit line while the adjustable resistancelocal bit line is set into a conducting state. The memory operationincludes applying the dummy word line voltage to the dummy word linewhile the adjustable resistance local bit line is set into theconducting state. The memory operation includes applying the selectedselect gate voltage to the select gate during the memory operation suchthat a current through the first memory cell does not exceed the maximumcurrent limit for the first memory cell.

One embodiment of the disclosed technology includes identifying a firstword line within a memory array and identifying a first global bit linewithin the memory array. The first global bit line is connected to anadjustable resistance bit line structure that includes an adjustableresistance local bit line and a select gate. A first memory cell isarranged between the adjustable resistance local bit line and the firstword line. The method further comprises determining a plurality ofunselected word line voltages to be applied to a plurality of unselectedword lines within the memory array. A plurality of unselected memorycells is arranged between the adjustable resistance local bit line andthe plurality of unselected word lines. The method further comprisesperforming a memory operation on the memory array. The memory operationincludes applying a selected word line voltage to the first word lineand applying a selected bit line voltage to the first global bit linewhile the adjustable resistance local bit line is set into a conductingstate. The memory operation includes applying the plurality ofunselected word line voltages to the plurality of unselected word lineswhile the adjustable resistance local bit line is set into theconducting state. In some cases, the memory operation comprises a RESEToperation and the determining a plurality of unselected word linevoltages includes determining the plurality of unselected word linevoltages such that the voltage stress across each of the plurality ofunselected memory cells is substantially 0V prior to the first memorycell being RESET. In some cases, the memory operation comprises a SEToperation and the determining a plurality of unselected word linevoltages includes determining the plurality of unselected word linevoltages such that the voltage stress across each of the plurality ofunselected memory cells is substantially 0V after the first memory cellis SET.

For purposes of this document, a first layer may be over or above asecond layer if zero, one, or more intervening layers are between thefirst layer and the second layer.

For purposes of this document, it should be noted that the dimensions ofthe various features depicted in the figures may not necessarily bedrawn to scale.

For purposes of this document, reference in the specification to “anembodiment,” “one embodiment,” “some embodiments,” or “anotherembodiment” may be used to describe different embodiments and do notnecessarily refer to the same embodiment.

For purposes of this document, a connection may be a direct connectionor an indirect connection (e.g., via another part). In some cases, whenan element is referred to as being connected or coupled to anotherelement, the element may be directly connected to the other element orindirectly connected to the other element via intervening elements. Whenan element is referred to as being directly connected to anotherelement, then there are no intervening elements between the element andthe other element.

For purposes of this document, the term “based on” may be read as “basedat least in part on.”

For purposes of this document, without additional context, use ofnumerical terms such as a “first” object, a “second” object, and a“third” object may not imply an ordering of objects, but may instead beused for identification purposes to identify different objects.

For purposes of this document, the term “set” of objects may refer to a“set” of one or more of the objects.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A method for operating a non-volatile memory, comprising: identifying a first word line within a memory array; identifying a first global bit line within the memory array, the first global bit line is connected to an adjustable resistance bit line structure, the adjustable resistance bit line structure includes an adjustable resistance local bit line and a select gate, a first memory cell is arranged between the adjustable resistance local bit line and the first word line; setting the adjustable resistance local bit line into a conducting state by applying a first voltage to the select gate; and performing a memory operation on the memory array, the memory operation includes applying a selected word line voltage to the first word line and a selected bit line voltage to the first global bit line while the adjustable resistance local bit line is set into the conducting state.
 2. The method of claim 1, wherein: the memory operation comprises a programming operation.
 3. The method of claim 1, wherein: the adjustable resistance local bit line comprises undoped polysilicon.
 4. The method of claim 1, wherein: the first memory cell comprises a ReRAM memory cell; and the memory array comprises a three-dimensional memory array.
 5. The method of claim 1, further comprising: identifying a second global bit line within the memory array, the second global bit line is connected to a second adjustable resistance bit line structure, the second adjustable resistance bit line structure includes a second adjustable resistance local bit line and a second select gate, a second memory cell is arranged between the second adjustable resistance local bit line and the first word line; and setting the second adjustable resistance local bit line into a non-conducting state by applying a second voltage different from the first voltage to the second select gate, the memory operation includes applying the selected word line voltage to the first word line while the adjustable resistance local bit line is set into the conducting state and the second adjustable resistance local bit line is set into the non-conducting state.
 6. The method of claim 1, further comprising: identifying a second global bit line within the memory array, the second global bit line is connected to a second adjustable resistance bit line structure, the second adjustable resistance bit line structure includes a second adjustable resistance local bit line and a second select gate, a second memory cell is arranged between the second adjustable resistance local bit line and the first word line; and setting the second adjustable resistance local bit line into a conducting state by applying the first voltage to the second select gate, the memory operation includes applying an unselected bit line voltage to the second global bit line while the adjustable resistance local bit line is set into the conducting state and the second adjustable resistance local bit line is set into the conducting state.
 7. The method of claim 1, wherein: the applying a first voltage to the select gate includes enabling a vertical thin-film transistor arranged above the select gate to pass the first voltage to the select gate.
 8. A non-volatile storage system, comprising: a memory array, the memory array includes a first intrinsic bit line and a second intrinsic bit line, the first intrinsic bit line connected to a first set of memory cells, the second intrinsic bit line connected a second set of memory cells; and one or more managing circuits in communication with the first intrinsic bit line and the second intrinsic bit line, the one or more managing circuits configured to set the first intrinsic bit line to a first resistance and the second intrinsic bit line to a second resistance less than the first resistance during a memory operation.
 9. The non-volatile storage system of claim 8, wherein: the first intrinsic bit line comprises a non-conductive bit line during the memory operation, the second intrinsic bit line comprises a conductive bit line during the memory operation.
 10. The non-volatile storage system of claim 8, wherein: the first intrinsic bit line comprises undoped polysilicon.
 11. The non-volatile storage system of claim 8, wherein: the first intrinsic bit line comprises a vertical intrinsic bit line that is arranged in a direction that is substantially orthogonal to a substrate.
 12. The non-volatile storage system of claim 8, wherein: the first set of memory cells includes ReRAM memory cells; the memory operation comprises a programming operation; and the memory array comprises a three-dimensional memory array.
 13. A non-volatile memory, comprising: a memory array, the memory array includes a first adjustable resistance bit line structure and a second adjustable resistance bit line structure, the first adjustable resistance bit line structure connected to a first set of memory cells, the second adjustable resistance bit line structure connected a second set of memory cells; and one or more managing circuits in communication with the first adjustable resistance bit line structure and the second adjustable resistance bit line structure, the one or more managing circuits configured to set the first adjustable resistance bit line structure into a conducting state and the second adjustable resistance bit line structure into a non-conducting state during a memory operation.
 14. The non-volatile memory of claim 13, wherein: the first adjustable resistance bit line structure includes a select gate surrounded by a layer of gate oxide, the layer of gate oxide surrounded by a layer of undoped polysilicon, the layer of undoped polysilicon surrounded by a layer of ReRAM material.
 15. The non-volatile memory of claim 13, wherein: the first adjustable resistance bit line structure includes a first semiconducting body region and a first select gate, the first semiconducting body region separated from the first select gate by a gate dielectric, the first semiconducting body region includes undoped polysilicon.
 16. The non-volatile memory of claim 15, wherein: the one or more managing circuits configured to apply a first voltage to the first select gate during the memory operation, the application of the first voltage to the first select gate causes a carrier concentration in the first semiconducting body region to increase.
 17. The non-volatile memory of claim 13, wherein: the first set of memory cells includes a first set of ReRAM memory cells; the second set of memory cells includes a second set of ReRAM memory cells; and the memory array comprises a three-dimensional memory array.
 18. The non-volatile memory of claim 13, wherein: the memory operation comprises one of a read operation or a programming operation.
 19. The non-volatile memory of claim 13, wherein: the one or more managing circuits identify a first word line within the memory array and identify a first global bit line within the memory array, the first global bit line is connected to the first adjustable resistance bit line structure, the first adjustable resistance bit line structure includes an adjustable resistance local bit line and a select gate, a first memory cell is arranged between the adjustable resistance local bit line and the first word line, the one or more managing circuits set the adjustable resistance local bit line into the conducting state by applying a first voltage to the select gate.
 20. The non-volatile memory of claim 19, wherein: the one or more managing circuits cause a selected word line voltage to be applied to the first word line and a selected bit line voltage to be applied to the first global bit line while the adjustable resistance local bit line is set into the conducting state. 